ES Bootstrap: Independent Groups
Directions for Use
Introduction
ES Bootstrap: Independent Groups is a windows-based program that provides
Bootstrapped confidence intervals for a robust variation of Cohen’s d measure of effectsize:⎛ Y −Y ⎞d R = .642 ⎜ t 2 t1 ⎟⎝ SW ⎠Formatting Data for Import
The imported data file must contain only numeric entries, and be in text format (have a“.txt” extension). Each row of the text file corresponds to a case, and each column
corresponds to a variable. One of three different delimiters may be used to delineate thevariables in the data base: a space, a comma, or a tab. For example, if our data fileconsists of three cases and five variables, then the data file could be space-delimited:1 1 1 1 1
2 2 2 2 2
3 3 3 3 3
Or comma-delimited:
1,1,1,1,1
2,2,2,2,2
3,3,3,3,3
Or tab-delimited:
123

12
3

1
2
3

1
2
3

123

The same delimiter must be used throughout the data file. In addition, the data must
begin in the first row. As a result, variable names should not be included in the data fileto be imported. ES Bootstrap will identify variables by their column placement.
ES Bootstrap can read in any number of variables and any number of cases. One of thevariables must be a variable indicated membership in one of at least two groups. A
second variable must be a dependent variable.
Importing Data
Directions for Using Robust ES Bootstrap: Independent Groups 1

The top portion of the main window of ES Bootstrap is devoted to importing the data file.
To import the data:(A) Specify the type of formatting used in delimiting the data file in the box on the far
left-hand side of the Main Window called “Type of Delimiter”. Three options areprovided: space-delimited, comma-delimited, and tab-delimited. The default
setting is space-delimited. If the appropriate type of delimiter is not provided, thedata will not be read in.
(B) Specify the directory (drive and folder) and the file name of the data file to beimported.
(C) Click the “Import Selected Data File” button towards the right-hand side of theMain Window. Upon clicking the “Import Selected Data File” button, a message
should appear in the Output Window that specifies the path of the selected file,the number of cases, and the number of variables.
For example, the message might look like:
Opened the text file: C:\Analysis\Data100.txt
Number of Cases: 200
Number of Variables: 2
Running Descriptive Analyses
To obtain descriptive statistics for each variable (e.g., mean, standard deviation, etc.),
select “Descriptives” from the Analyses Menu, and the select all desired variables. The
results will be printed in the Output Window.
Construct Bootstrapped Confidence Intervals
To obtain bootstrapped confidence intervals for the effect size estimator, do thefollowing:(A) Select “Effect Size Bootstrap” from the Analyses Menu. This will result in theappearance of the Effect Size Bootstrap Window.
(B) Select the dependent variables in the “Pick Dependent Variable” list.
(C) Select the grouping variable in the “Pick Group Variable ” list.
(D) Indicate which value for the grouping variable indicates group 1 and which value
indicates group 2.
(E) Select the number of bootstrap trials (from 600 to 2000). The current trial number
will be displayed in the “Trial number” box during the analysis.
Directions for Using Robust ES Bootstrap: Independent Groups 2

(F) Select the level of confidence desired (90%, 95%, 99%).
For example, running the bootstrapped confidence intervals with 600 trials and a 95%
confidence interval yields the following output:
GROUP-LEVEL STATISTICS--------------------------------------------Statistic
Group 1Group 2--------------------------------------------Sample Size
100100
Mean
Trimmed Mean

1.63
1.533

2.03
2.067

Variance
1.427
0.736
Wins. Var.
0.789
0.463--------------------------------------------Dep Var = Var 2; Grp Var = Var 1

EFFECT SIZE ESTIMATES AND 95% CI-------------------------------------------------Pooled variance = 1.082
Pooled Winsorized variance = 0.626
Cohen's D = 0.385
Robust Cohen's D = 0.433
95% CI for population D = (0.15, 0.706)
95% CI for population Robust D = (0.192, 0.827)-------------------------------------------------Number of attempted bootstrap trials = 600
Number of valid D bootstrap trials = 600
Number of valid robust D bootstrap trials = 600

Note that the appearance of “9999999” for any of the statistics values indicates that thestatistic could not be computed (usually due to lack of variance).
Save Output
To save the contents of the Output Window, select “Save Output” from the File Menu. A
window will appear that will permit you to select the directory where you wish to savethe output. The output file will be saved as a text document (with extension “.txt”).
Open Output
You may open the output of a previous ES Bootstrap session in the Output Window ofES Bootstrap by selecting “Open Output” from the File Menu. Note that opening anoutput file in ES Bootstrap will cause any output currently in the Output Window to be
replaced by the contents of the opened file.

Directions for Using Robust ES Bootstrap: Independent Groups 3