# Independent Samples Test Worksheet

Statistics for the Behavioral SciencesSpring 2022

PSYCH-UA.10

Computer Assignment #3: Check syllabus for due date

Answer the following questions using SPSS and some computations by hand. Please make clear

which SPSS output goes with which question. Only include necessary output. Use the notes from

lab to help you. Remember, even if you have a large sample, you will be doing a t-test in SPSS,

not a z-test.

1) Open data labeled Cosmetic Surgery Revised in Brightspace (Content > SPSS

Assignments > SPSS Assignment Data > Cosmetic Surgery Revised B). In the US,

there was a 1600% increase in cosmetic surgical and non-surgical treatments between

1992 and 2002. According to researchers, there are two main reasons to have cosmetic

surgery: 1) to help with a physical problem and 2) to change one’s external appearance.

This data looks at whether there is a difference in Quality of Life After Surgery between

people who elect to undergo cosmetic surgery for a physical problem and those who want

to change their external appearance. The two variables you need are Reason (reason for

surgery) and Quality of Life After Surgery (Post_Qol); higher numbers mean participants

experience a better quality of life.

Only using SPSS, calculate a t-test and the 95% CI for the difference in Quality of Life

After Surgery between people who undergo the surgery for a physical problem and those

who want to change their appearance, just as you did in lab. Select the Group Statistics

box and the Independent Samples Test (t-test portion) for your output. Answer the

following questions:

a) In symbols and words, what are the null and alternative hypotheses for this

experiment?

b) What is the standard error of the difference between the means (assume equal

variances for this question)?

c) Using Levene’s test, do we have homogeneity of variance? How do we know?

d) What is the conclusion (assume equal variances for this question)? Write up the

results in APA format.

e) What is the 95% confidence interval for these data (assume equal variances for

this question)? How would you determine significance using this interval?

f) Explain what a Type I and Type II error would be for this study. Which one did

we possibly make?

Output should include the Independent Samples Test and Group Statistics

boxes. Just to make sure you’re on the right track, the mean difference

should equal .7825 and the sample sizes should be 36 people who wanted to

change their appearance and 84 people who had a physical reason for their

surgery.

2) Researchers would like to investigate the Baseline Quality of Life of these patients for the

different groups. Before they analyze their data, they would like to do a power analysis.

The descriptives for the variable “Baseline Quality of Life” are: M1 = 66.0, s12 = 45.0,

n1 = 36 for people who chose surgery to change their appearance and M 2 = 63.0,

s22 = 48.0, n2 = 84 for those who choose surgery for a physical reason.

a) Use these descriptive statistics to find the power for this study. Please show your

hand calculations: First, find s2pooled. Then find g (the sample estimate of d remember to take the square root of s2pooled). Always use the absolute value of g

(or d) for these calculations. Find δ, remembering to replace d with g. Find the

harmonic mean of the n’s because you have unequal sample sizes. Assuming

α = .05 (two tailed), use the table to find the power.

b) What is the probability of a Type II error for this study?

c) Using the formula to find n, what sample size would you have needed for a

power of .90? Remember to use g instead of d (absolute value).

3) Open data labeled Assignment NELS B in Brightspace (Content > SPSS Assignments >

SPSS Assignment Data > Assignment NELS B). This data consists of measures of

achievement outcomes in four core subject areas (English, history, mathematics and

science), in addition to personal, familial, social, institutional, and cultural factors that

might relate to these outcomes. We are specifically interested in the relationships

between tenth grade self-concept, teacher interest, socioeconomic status, and tenth grade

measures of academic achievement in math, social studies and science. The variables you

want are:

Slfcnc10 (tenth grade self-concept)

SES (socioeconomic status)

Tcherint (teacher interest) Even though this variable is technically an ordinal scale, we are

treating here as a ratio scale

Achsls10 tenth grade social studies achievement)

Achmat10 (tenth grade math achievement)

Achsci10 (tenth grade science achievement)

a) Calculate correlations in SPSS (Analyze>Correlate>Bivariate) for all of the

variables. Print out the box of correlations.

b) Using only SPSS, how many correlations are significant? Between which two

variables is the highest correlation?

c) Write up the results in APA format for the correlations between socioeconomic

status and 10th grade math achievement, teacher interest and 10 th grade math

achievement, and 10th grade science achievement and 10th grade math

achievement (even if they are not significant write them in APA format).

d) Out of all the variables for this problem, which is the best predictor of 10 th grade

math achievement? Why?

e) Create and print an interactive scatterplot (Graphs > Chart Builder > Simple

Scatterplot) of the variables socioeconomic status (on the X-axis) and 10 th grade

math achievement (on the Y-axis). Describe the pattern of data.

f) What happens if the range is restricted to only students that score at or above an

SES of 20: Go to Data>Select Cases>If condition is satisfied. Move SES from

the left-hand box to the formula box (using the arrow). Then put ≥ 20, so that

the selection looks like this: SES >= 20. Click continue, then OK. This should

select only those SES scores at or above 20.

g) Find the correlation between SES and 10th grade math achievement the same way

as before (Analyze>Correlate>Bivariate) for just these two variables. (Notice that

N will now equal 172). Print out this box of correlations. Also create and print

out a new scatterplot.

h) What is the correlation now? Why is it different from before?

Output should include two correlation boxes and two scatterplots. The N for SES and 10 th

grade math achievement should be 399 (before the restriction of range).

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A One-Stop-Shop for Nailing Your SPSS Assignments

2

SPSS Grading Guide

●

We are using Gradescope for all assignments. All grading rubrics will appear on Gradescope as

you submit your assignment

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Not typed (-1 off overall grade) (this does not apply to formulas or hand calculations, although

typed is preferable)

o

●

Although typed makes it easier for us to read, the actual reason why we require the assignment to

be typed is because the intention of this course is to prepare students to be able to conduct and

report statistics in the future, should they need to, and in the real world they would never

handwrite a report (for statistics or any other course).

Output printed directly from SPSS; it should be copied to a word document (-2 off overall grade)

o

This is because the SPSS assignments are also assessing students’ ability to differentiate output,

so including all output (even if not requested) does not demonstrate this ability – it would be akin

selecting all multiple choice answers.

▪

●

Extra output is included (-1 off overall grade)

o

●

Similar to previous points about how you present output in reports

Critical value is included in addition to test statistic (-.25 each occurrence)

o

●

Similar to above point for typing; they should be properly reported and so tables shouldn’t be

wrapped or split within/across pages (unless the amount of output justifies it, which, for this

course, it doesn’t)

Graphs not properly titled (-.1 for each occurrence)

o

●

See above

Split tables (-.2 for each occurrence)

o

●

We usually only indicate -2 on the first assignment (which is where points aren’t actually

taken off), and tend to only end up subtracting up to 1pt since usually there’s not an

egregious amount of additional output (although there are the occasional cases)

This is essentially like circling two multiple choice answers – as if you don’t know which is

correct and are hoping to get credit based on one being right.

Hand-calculating a value that should have been retrieved from SPSS output (-.2 for each

occurrence)

o

These are SPSS assignments after all, so unless the question specifies to calculate a value by

hand (which some do) you should be finding the answers via SPSS

▪

Grading this way is also consistent with exams in which some questions require answers

only using SPSS output or else points are deducted

3

The output is clearly

sectioned by question.

There’s no extraneous output

such as syntax or

unrequested tables/charts.

SPSS Output

4

Question 1

Question 2

Both graphs have informative

titles that include the relevant

variable(s).

Output has been resized so that

there are multiple tables and

graphs on this one page. Narrow

margins are acceptable and may

help you fit more output

efficiently.

However, all output must

remain legible, with nothing cut

off of the page.

Independent Samples Test

Levene’s Test for

Equality of

Variances

F

Perceived

Choice

subscale of

Intrinsic

Equal

variance

s

assumed

.08

2

Sig.

.775

t-test for Equality of Means

t

2.9

12

df

Sig.

(2taile

d)

245

.004

Mean

Differe

nce

-.4628

7

Std.

Error

Differe

nce

.15897

95%

Confidence

Interval of the

Difference

Low

er

Upper

-.77

599

-.149

75

5

Motivation

Inventory

Equal

variance

s not

assumed

2.9

00

236.

631

.004

-.4628

7

.15961

-.77

732

-.148

43

Written Responses

Question 1

a) I am now answering the question in sentence form. Blah blah blah blah blah. Words and words and words

and words and words.

b) This is my answer to another part of question 1. Text text text text.

Question 2

a) I’m answering this question, now, also in sentence form. Blah blah blah blah blah. Words and words and

words and words and words.

b) This is my answer to part b of question 2. Text text text text.

As you can see, this organization follows a logical

and consistent pattern, with question #s clearly

labeled.

All written responses are typed—never handwritten—so as to resemble a formal report.

Other options for organizing your assignments:

You could lead with the written responses, then include your SPSS

Output section.

Instead of separating out the SPSS output, you could simply organize

by Question #:

Question 1

Written Response

SPSS Output

Blank space to hand-write in

formulas/hand-calculations, if applicable

Question 2

Written Response

SPSS Output

Blank space to hand-write in

6

Academic Integrity

Each assignment must be your original work!

• Absolutely NO sharing of outputs

• ALL written work must be done independently

Any plagiarism of another’s assignment will result in a ZERO for the assignment and appropriate

disciplinary action at the instructors ’discretion.

Your understanding of this is attested to in the header included in every assignment (see page 2).

What counts as plagiarism?

• “Collaborating between two or more students who then submit the same paper under their individual

names.”

• “Submitting another student’s work with your name on it.”

• “Paraphras[ing] or restate[ing] someone else’s facts, analysis and/or conclusions.”

Bottom line: It’s not worth the risk!

● Grading is done by experienced TAs who are able to spot tell-tale signs of copying.

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Miscellaneous Tips

•

Due dates are listed on the syllabus, so plan ahead

•

Lab worksheets guide you step-by-step through the analyses you’ll need to run for assignments

•

Check that you entered data correctly

•

Writing in black ink is preferred

•

Make sure you answer ALL parts of ALL questions

•

Do SPSS problems in the order on the assignment and label each answer

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Title & label graphs

•

Box or highlight every answer to hand-calculations

•

Only include relevant output

•

Resize output & arrange in Word document to reduce excess pages and avoid splitting charts/tables

between pages. Save this as a pdf for Gradescope.

•

Double-check output with a classmate’s if possible. This way, you can troubleshoot in case of

discrepancies and avoid major point losses. (However, see info on academic integrity.)

•

See “Rules” on pages 3-4 for additional tips!