### Archive

Archive for September 2013

## Export tables to Excel

There is a new command in Stata 13, putexcel, that allows you to easily export matrices, expressions, and stored results to an Excel file. Combining putexcel with a Stata command’s stored results allows you to create the table displayed in your Stata Results window in an Excel file.

A stored result is simply a scalar, macro, or matrix stored in memory after you run a Stata command. The two main types of stored results are e-class (for estimation commands) and r-class (for general commands). You can list a command’s stored results after it has been run by typing ereturn list (for estimation commands) and return list (for general commands). Let’s try a simple example by loading the auto dataset and running correlate on the variables foreign and mpg

. sysuse auto
(1978 Automobile Data)

. correlate foreign mpg
(obs=74)

|  foreign      mpg
-------------+------------------
foreign |   1.0000
mpg |   0.3934   1.0000


Because correlate is not an estimation command, use the return list command to see its stored results.

. return list

scalars:
r(N) =  74
r(rho) =  .3933974152205484

matrices:
r(C) :  2 x 2


Now we can use putexcel to export these results to Excel. The basic syntax of putexcel is

putexcel excel_cell=(expression) … using filename [, options]

If you are working with matrices, the syntax is

putexcel excel_cell=matrix(expression) … using filename [, options]

It is easy to build the above syntax in the putexcel dialog. There is a helpful video on Youtube about the dialog here. Let’s list the matrix r(C) to see what it contains.

. matrix list r(C)

symmetric r(C)[2,2]
foreign        mpg
foreign          1
mpg  .39339742          1


To re-create the table in Excel, we need to export the matrix r(C) with the matrix row and column names. The command to type in your Stata Command window is

putexcel A1=matrix(r(C), names) using corr


Note that to export the matrix row and column names, we used the names option after we specifed the matrix r(C). When I open the file corr.xlsx in Excel, the table below is displayed.

Next let’s try a more involved example. Load the auto dataset, and run a tabulation on the variable foreign. Because tabulate is not an estimation command, use the return list command to see its stored results.

. sysuse auto
(1978 Automobile Data)

. tabulate foreign

Car type |      Freq.     Percent        Cum.
------------+-----------------------------------
Domestic |         52       70.27       70.27
Foreign |         22       29.73      100.00
------------+-----------------------------------
Total |         74      100.00

. return list

scalars:
r(N) =  74
r(r) =  2


tabulate is different from most commands in Stata in that it does not automatically save all the results we need into the stored results (we will use scalar r(N)). We need to use the matcell() and matrow() options of tabulate to save the results produced by the command into two Stata matrices.

. tabulate foreign, matcell(freq) matrow(names)

Car type |      Freq.     Percent        Cum.
------------+-----------------------------------
Domestic |         52       70.27       70.27
Foreign |         22       29.73      100.00
------------+-----------------------------------
Total |         74      100.00

. matrix list freq

freq[2,1]
c1
r1  52
r2  22

. matrix list names

names[2,1]
c1
r1   0
r2   1


The putexcel commands used to create a basic tabulation table in Excel column 1 row 1 are

putexcel A1=("Car type") B1=("Freq.") C1=("Percent") using results, replace
putexcel A2=matrix(names) B2=matrix(freq) C2=matrix(freq/r(N)) using results,
modify


Below is the table produced in Excel by these commands.

Again this is a basic tabulation table. You probably noticed that we did not have the Cum. column or the Total row in the export table. Also our Car type column contains the numeric values (0,1), not the value lables (Domestic, Foreign) of the variable foreign, and our Percent column is not formatted correctly. To get the exact table displayed in the Results window into an Excel file takes a little programming. With a few functions and a forvalues loop, we can easily export any table produced by running the tabulate command on a numeric variable.

There are two extended macro functions, label and display, that can help us. The label function can extract the value labels for each variable, and the display function can correctly format numbers for our numeric columns. Last, we use forvalues to loop over the rows of the returned matrices to produce our final tables. Our do-file to produce the tabulate table in Excel looks like

sysuse auto
tabulate foreign, matcell(freq) matrow(names)

putexcel A1=("Car type") B1=("Freq.") C1=("Percent") D1=("Cum.") using results, replace

local rows = rowsof(names)
local row = 2
local cum_percent = 0

forvalues i = 1/rows' {

local val = names[i',1]
local val_lab : label (foreign) val'

local freq_val = freq[i',1]

local percent_val = freq_val'/r(N)'*100
local percent_val : display %9.2f percent_val'

local cum_percent : display %9.2f (cum_percent' + percent_val')

putexcel Arow'=("val_lab'") Brow'=(freq_val') Crow'=(percent_val') ///
Drow'=(cum_percent') using results, modify
local row = row' + 1
}

putexcel Arow'=("Total") Brow'=(r(N)) Crow'=(100.00) using results, modify


The above commands produce this table in Excel:

The solution above works well for this one table, but what if we need to export the tabulation table for 100 variables to the same Excel spreadsheet? It would be very tedious to run the same do-file 100 times, each time changing the cell and row numbers. Now we could easily change our do-file into the Stata command (ado-file) called tab2xl. The syntax for our new command could be

tab2xl varname using filename, row(rownumber) col(colnumber) [replace sheet(name)]


The pseudocode of our program (file tab2xl.ado) looks like

program tab2xl
/* parse command syntax */

/* tabulate varname */

/* get column letters based on starting column number passed in */

/* write header row to filename in starting row number passed in */

/* loop over rows of returned matrix and calculate/write values to filename */

/* write total row to filename */
end


If you would like to download a working version of our tab2xl command, type

net install http://www.stata.com/users/kcrow/tab2xl


in Stata.

Categories: Programming Tags:

## Measures of effect size in Stata 13

Today I want to talk about effect sizes such as Cohen’s d, Hedges’s g, Glass’s Δ, η2, and ω2. Effects sizes concern rescaling parameter estimates to make them easier to interpret, especially in terms of practical significance.

Many researchers in psychology and education advocate reporting of effect sizes, professional organizations such as the American Psychological Association (APA) and the American Educational Research Association (AERA) strongly recommend their reporting, and professional journals such as the Journal of Experimental Psychology: Applied and Educational and Psychological Measurement require that they be reported.

Anyway, today I want to show you

1. What effect sizes are.
2. How to calculate effect sizes and their confidence intervals in Stata.
3. How to calculate bootstrap confidence intervals for those effect sizes.
4. How to use Stata’s effect-size calculator.

## 1. What are effect sizes?

The importance of research results is often assessed by statistical significance, usually that the p-value is less than 0.05. P-values and statistical significance, however, don’t tell us anything about practical significance.

What if I told you that I had developed a new weight-loss pill and that the difference between the average weight loss for people who took the pill and the those who took a placebo was statistically significant? Would you buy my new pill? If you were overweight, you might reply, “Of course! I’ll take two bottles and a large order of french fries to go!”. Now let me add that the average difference in weight loss was only one pound over the year. Still interested? My results may be statistically significant but they are not practically significant.

Or what if I told you that the difference in weight loss was not statistically significant — the p-value was “only” 0.06 — but the average difference over the year was 20 pounds? You might very well be interested in that pill.

The size of the effect tells us about the practical significance. P-values do not assess practical significance.

All of which is to say, one should report parameter estimates along with statistical significance.

In my examples above, you knew that 1 pound over the year is small and 20 pounds is large because you are familiar with human weights.

In another context, 1 pound might be large, and in yet another, 20 pounds small.

Formal measures of effects sizes are thus usually presented in unit-free but easy-to-interpret form, such as standardized differences and proportions of variability explained.

### The “d” family

Effect sizes that measure the scaled difference between means belong to the “d” family. The generic formula is

The estimators differ in terms of how sigma is calculated.

Cohen’s d, for instance, uses the pooled sample standard deviation.

Hedges’s g incorporates an adjustment which removes the bias of Cohen’s d.

Glass’s Δ was originally developed in the context of experiments and uses the “control group” standard deviation in the denominator. It has subsequently been generalized to nonexperimental studies. Because there is no control group in observational studies, Kline (2013) recommends reporting Glass’s Δ using the standard deviation for each group. Glass’s Delta_1 uses one group’s standard deviation and Delta_2 uses the other group’s.

Although I have given definitions to Cohen’s d, Hedges’s g, and Glass’s Δ, different authors swap the definitions around! As a result, many authors refer to all of the above as just Delta.

Be careful when using software to know which Delta you are getting. I have used Stata terminology, of course.

Anyway, the use of a standardized scale allows us to assess of practical significance. Delta = 1.5 indicates that the mean of one group is 1.5 standard deviations higher than that of the other. A difference of 1.5 standard deviations is obviously large, and a difference of 0.1 standard deviations is obviously small.

### The “r” family

The r family quantifies the ratio of the variance attributable to an effect to the total variance and is often interpreted as the “proportion of variance explained”. The generic estimator is known as eta-squared,

η2 is equivalent to the R-squared statistic from linear regression.

ω2 is a less biased variation of η2 that is equivalent to the adjusted R-squared.

Both of these measures concern the entire model.

Partial η2 and partial ω2 are like partial R-squareds and concern individual terms in the model. A term might be a variable or a variable and its interaction with another variable.

Both the d and r families allow us to make an apples-to-apples comparison of variables measured on different scales. For example, an intervention could affect both systolic blood pressure and total cholesterol. Comparing the relative effect of the intervention on the two outcomes would be difficult on their original scales.

How does one compare mm/Hg and mg/dL? It is straightforward in terms of Cohen’s d or ω2 because then we are comparing standard deviation changes or proportion of variance explained.

## 2. How to calculate effect sizes and their confidence intervals in Stata

Consider a study where 30 school children are randomly assigned to classrooms that incorporated web-based instruction (treatment) or standard classroom environments (control). At the end of the school year, the children were given tests to measure reading and mathematics skills. The reading test is scored on a 0-15 point scale and, the mathematics test, on a 0-100 point scale.

Let’s download a dataset for our fictitious example from the Stata website by typing:

. use http://www.stata.com/videos13/data/webclass.dta

Contains data from http://www.stata.com/videos13/data/webclass.dta
obs:            30                          Fictitious web-based learning
experiment data
vars:             5                          5 Sep 2013 11:28
size:           330                          (_dta has notes)
-------------------------------------------------------------------------------
storage   display    value
variable name   type    format     label      variable label
-------------------------------------------------------------------------------
id              byte    %9.0g                 ID Number
treated         byte    %9.0g      treated    Treatment Group
agegroup        byte    %9.0g      agegroup   Age Group
reading         float   %9.0g                 Reading Score
math            float   %9.0g                 Math Score
-------------------------------------------------------------------------------

. notes

_dta:
1.  Variable treated records 0=control, 1=treated.
2.  Variable agegroup records 1=7 years old, 2=8 years old, 3=9 years old.


We can compute a t-statistic to test the null hypothesis that the average math scores are the same in the treatment and control groups.

. ttest math, by(treated)

Two-sample t test with equal variances
------------------------------------------------------------------------------
Group |     Obs        Mean    Std. Err.   Std. Dev.   [95% Conf. Interval]
---------+--------------------------------------------------------------------
Control |      15    69.98866    3.232864    12.52083    63.05485    76.92246
Treated |      15    79.54943    1.812756    7.020772    75.66146     83.4374
---------+--------------------------------------------------------------------
combined |      30    74.76904    2.025821    11.09588    70.62577    78.91231
---------+--------------------------------------------------------------------
diff |           -9.560774    3.706412               -17.15301   -1.968533
------------------------------------------------------------------------------
diff = mean(Control) - mean(Treated)                          t =  -2.5795
Ho: diff = 0                                     degrees of freedom =       28

Ha: diff < 0                 Ha: diff != 0                 Ha: diff > 0
Pr(T < t) = 0.0077         Pr(|T| > |t|) = 0.0154          Pr(T > t) = 0.9923


The treated students have a larger mean, yet the difference of -9.56 is reported as negative because -ttest- calculated Control minus Treated. So just remember, negative differences mean Treated > Control in this case.

The t-statistic equals -2.58 and its two-sided p-value of 0.0154 indicates that the difference between the math scores in the two groups is statistically significant.

Next, let’s calculate effect sizes from the d family:

. esize twosample math, by(treated) cohensd hedgesg glassdelta

Effect size based on mean comparison

Obs per group:
Control =     15
Treated =     15
---------------------------------------------------------
Effect Size |   Estimate     [95% Conf. Interval]
--------------------+------------------------------------
Cohen's d |  -.9419085    -1.691029   -.1777553
Hedges's g |   -.916413    -1.645256   -.1729438
Glass's Delta 1 |  -.7635896     -1.52044    .0167094
Glass's Delta 2 |  -1.361784    -2.218342   -.4727376
---------------------------------------------------------


Cohen’s d and Hedges’s g both indicate that the average reading scores differ by approximately -0.93 standard deviations with 95% confidence intervals of (-1.69, -0.18) and (-1.65, -0.17) respectively.

Since this is an experiment, we are interested in Glass’s Delta 1 because it is calculated using the control group standard deviation. Average reading scores differ by -0.76 and the confidence interval is (-1.52, 0.02).

The confidence intervals for Cohen’s d and Hedges’s g do not include the null value of zero but the confidence interval for Glass’s Delta 1 does. Thus we cannot completely rule out the possibility that the treatment had no effect on math scores.

Next we could incorporate the age group of the children into our analysis by using a two-way ANOVA to test the null hypothesis that the mean math scores are equal for all groups.

. anova math treated##agegroup

Number of obs =      30     R-squared     =  0.2671
Root MSE      = 10.4418     Adj R-squared =  0.1144

Source |  Partial SS    df       MS           F     Prob > F
-----------------+----------------------------------------------------
Model |  953.697551     5   190.73951       1.75     0.1617
|
treated |  685.562956     1  685.562956       6.29     0.0193
agegroup |  47.7059268     2  23.8529634       0.22     0.8051
treated#agegroup |  220.428668     2  110.214334       1.01     0.3789
|
Residual |  2616.73825    24  109.030761
-----------------+----------------------------------------------------
Total |   3570.4358    29  123.118476


The F-statistic for the entire model is not statistically significant (F=1.75, ndf=5, ddf=24, p=0.1617) but the F-statistic for the main effect of treatment is statistically significant (F=6.29, ndf=1, ddf=24, p=0.0193).

We can compute the η2 and partial η2 estimates for this model using the estat esize command immediately after our anova command (note that estat esize works after the regress command too).

. estat esize

Effect sizes for linear models

---------------------------------------------------------------------
Source |   Eta-Squared     df     [95% Conf. Interval]
----------------------+----------------------------------------------
Model |   .2671096         5            0    .4067062
|
treated |   .2076016         1     .0039512    .4451877
agegroup |   .0179046         2            0    .1458161
treated#agegroup |   .0776932         2            0     .271507
---------------------------------------------------------------------


The overall η2 indicates that our model accounts for approximately 26.7% of the variablity in math scores though the 95% confidence interval includes the null value of zero (0.00%, 40.7%). The partial η2 for treatment is 0.21 (21% of the variability explained) and its 95% confidence interval excludes zero (0.3%, 20%).

We could calculate the alternative r-family member ω2 rather than η2 by typing

. estat esize, omega

Effect sizes for linear models

---------------------------------------------------------------------
Source | Omega-Squared     df     [95% Conf. Interval]
----------------------+----------------------------------------------
Model |   .1144241         5            0    .2831033
|
treated |    .174585         1            0    .4220705
agegroup |          0         2            0    .0746342
treated#agegroup |   .0008343         2            0    .2107992
---------------------------------------------------------------------


The overall ω2 indicates that our model accounts for approximately 11.4% of the variability in math scores and treatment accounts for 17.5%. This perplexing result stems from the way that ω2 and partial ω2 are calculated. See Pierce, Block, & Aguinis (2004) for a thorough explanation.

Except for the η2 for treatment, the confidence intervals include 0 so we cannot rule out the possibility that there is no effect. Whether results are practically significant is generically a matter context and opinion. In some situations, accounting for 5% of the variability in an outcome could be very important and in other situations accounting for 30% may not be.

We could repeat the same analyses for the reading scores using the following commands:

. ttest reading, by(treated)
. esize twosample reading, by(treated) cohensd hedgesg glassdelta
. anova reading treated##agegroup
. estat esize
. estat esize, omega


None of the t- or F-statistics for reading scores were statistically significant at the 0.05 level.

Even though the reading and math scores were measured on two different scales, we can directly compare the relative effect of the treatment using effect sizes:

        Effect Size   |     Reading Score          Math Score
------------------------------------------------------------
Cohen's d     |   -0.23 (-0.95 - 0.49)  -0.94 (-1.69 - -0.18)
Hedges's g    |   -0.22 (-0.92 - 0.48)  -0.92 (-1.65 - -0.17)
Glass's Delta |   -0.21 (-0.93 - 0.51)  -0.76 (-1.52 -  0.02)
Eta-squared   |    0.02 ( 0.00 - 0.20)   0.21 ( 0.00 -  0.44)
Omega-squared |    0.00 ( 0.00 - 0.17)   0.17 ( 0.00 -  0.42)


The results show that the average reading scores in the treated and control groups differ by approximately 0.22 standard deviations while the average math scores differ by approximately 0.92 standard deviations. Similarly, treatment status accounted for almost none of the variability in reading scores while it accounted for roughly 17% of the variability in math scores. The intervention clearly had a larger effect on math scores than reading scores. We also know that we cannot completely rule out an effect size of zero (no effect) for both reading and math scores because several confidence intervals included zero. Whether or not the effects are practically significant is a matter of interpretation but the effect sizes provide a standardized metric for evaluation.

## 3. How to calculate bootstrap confidence intervals

Simulation studies have shown that bootstrap confidence intervals for the d family may be preferable to confidence intervals based on the noncentral t distribution when the variable of interest does not have a normal distribution (Kelley 2005; Algina, Keselman, and Penfield 2006). We can calculate bootstrap confidence intervals for Cohen’s d and Hedges’s g using Stata’s bootstrap prefix:

. bootstrap r(d) r(g), reps(500) nowarn:  esize twosample reading, by(treated)
(running esize on estimation sample)

Bootstrap replications (500)
----+--- 1 ---+--- 2 ---+--- 3 ---+--- 4 ---+--- 5
..................................................    50
..................................................   100
..................................................   150
..................................................   200
..................................................   250
..................................................   300
..................................................   350
..................................................   400
..................................................   450
..................................................   500

Bootstrap results                               Number of obs      =        30
Replications       =       500

command:  esize twosample reading, by(treated)
_bs_1:  r(d)
_bs_2:  r(g)

------------------------------------------------------------------------------
|   Observed   Bootstrap                         Normal-based
|      Coef.   Std. Err.      z    P>|z|     [95% Conf. Interval]
-------------+----------------------------------------------------------------
_bs_1 |   -.228966   .3905644    -0.59   0.558    -.9944582    .5365262
_bs_2 |  -.2227684   .3799927    -0.59   0.558    -.9675403    .5220036
------------------------------------------------------------------------------


The bootstrap estimate of the 95% confidence interval for Cohen’s d is -0.99 to 0.54 which is slightly wider than the earlier estimate based on the non-central t distribution (see [R] esize for details). The bootstrap estimate is slightly wider for Hedges’s g as well.

## 4. How to use Stata’s effect-size calculator

You can use Stata’s effect size calculators to estimate them using summary statistics. If we know that the mean, standard deviation and sample size for one group is 70, 12.5 and 15 respectively and 80, 7 and 15 for another group, we can use esizei to estimate effect sizes from the d family:

. esizei 15 70 12.5 15 80 7, cohensd hedgesg glassdelta

Effect size based on mean comparison

Obs per group:
Group 1 =     15
Group 2 =     15
---------------------------------------------------------
Effect Size |   Estimate     [95% Conf. Interval]
--------------------+------------------------------------
Cohen's d |  -.9871279    -1.739873   -.2187839
Hedges's g |  -.9604084    -1.692779   -.2128619
Glass's Delta 1 |        -.8    -1.561417   -.0143276
Glass's Delta 2 |  -1.428571    -2.299112   -.5250285
---------------------------------------------------------


We can estimate effect sizes from the r family using esizei with slightly different syntax. For example, if we know the numerator and denominator degrees of freedom along with the F statistic, we can calculate η2 and ω2 using the following command:

. esizei 1 28 6.65

Effect sizes for linear models

---------------------------------------------------------
Effect Size |   Estimate     [95% Conf. Interval]
--------------------+------------------------------------
Eta-Squared |   .1919192     .0065357    .4167874
Omega-Squared |   .1630592            0    .3959584
---------------------------------------------------------
`

## Video demonstration

Stata has dialog boxes that can assist you in calculating effect sizes. If you would like a brief introduction using the GUI, you can watch a demonstration on Stata’s YouTube Channel:

Tour of effect sizes in Stata

## Final thoughts and further reading

Most older papers and many current papers do not report effect sizes. Nowadays, the general consensus among behavioral scientists, their professional organizations, and their journals is that effect sizes should always be reported in addition to tests of statistical significance. Stata 13 now makes it easy to compute most popular effects sizes.

Some methodologists believe that effect sizes with confidence intervals should always be reported and that statistical hypothesis tests should be abandoned altogether; see Cumming (2012) and Kline (2013). While this may sound like a radical notion, other fields such as epidemiology have been moving in this direction since the 1990s. Cumming and Kline offer compelling arguments for this paradigm shift as well as excellent introductions to effect sizes.

American Psychological Association (2009). Publication Manual of the American Psychological Association, 6th Ed. Washington, DC: American Psychological Association.

Algina, J., H. J. Keselman, and R. D. Penfield. (2006). Confidence interval coverage for Cohen’s effect size statistic. Educational and Psychological Measurement, 66(6): 945–960.

Cumming, G. (2012). Understanding the New Statistics: Effect Sizes, Confidence Intervals, and Meta-Analysis. New York: Taylor & Francis.

Kelley, K. (2005). The effects of nonnormal distributions on confidence intervals around the standardized mean difference: Bootstrap and parametric confidence intervals. Educational and Psychological Measurement 65: 51–69.

Kirk, R. (1996). Practical significance: A concept whose time has come. Educational and Psychological Measurement, 56, 746-759.

Kline, R. B. (2013). Beyond Significance Testing: Statistics Reform in the Behavioral Sciences. 2nd ed. Washington, DC: American Psychological Association.

Pierce, C.A., Block, R. A., and Aguinis, H. (2004). Cautionary note on reporting eta-squared values from multifactor ANOVA designs. Educational and Psychological Measurement, 64(6) 916-924

Thompson, B. (1996) AERA Editorial Policies regarding Statistical Significance Testing: Three Suggested Reforms. Educational Researcher, 25(2) 26-30

Wilkinson, L., & APA Task Force on Statistical Inference. (1999). Statistical methods in psychology journals: Guidelines and explanations. American Psychologist, 54, 594-604

Categories: Statistics Tags: