In a study of the effects of alcohol on sperm cells a research collected 2 sperm
ID: 3363274 • Letter: I
Question
In a study of the effects of alcohol on sperm cells a research collected 2 sperm samples from a group of volunteers. One sperm sample was collected after a 2-week period when no alcohol was consumed, and the second sperm sample was collected after a 2-week period in which the participants had consumed 3 beers per day. The order of treatments was counterbalanced across the participants. The percentage of sperm cells that contained genetic defects was recorded. The results are below. Set the alpha at 0.05
no alcohol
Alcohol
23
29
18
24
22
27
24
30
9
13
16
18
17
14
32
36
20
22
26
23
You will have to determine whether to use a Student’s t test (between subjects design) or a paired t test (repeated measures design).
Use the alpha level stated in the question, including whether the hypothesis being tested is 1-tailed or 2-tailed.
Use Table A.3 p.409 of Jackson to identify the critical t value for these data and hypothesis (again, make sure you have correctly determined whether the hypothesis requires a 1-tailed or a 2-tailed test.
If the t statistic is significant, calculate the effect size (Cohen’s d) and state whether the effect size is small, medium or large.
Copy the SPSS t statistic output and paste it into your word document.
Plot the means using excel (cut and paste into the word document)
Provide a written description of the results as they pertain to the hypothesis being tested. Describe the results in APA format: i.e., give a verbal description of the effect and include the t statistic in parenthesis (e.g., t(df) = x.yy, p<.05 Cohen’s d = XX).
no alcohol
Alcohol
23
29
18
24
22
27
24
30
9
13
16
18
17
14
32
36
20
22
26
23
Explanation / Answer
Independent Samples t test 2-tailed
H0: 1 - 2 = 0 i.e. (1 = 2)
H1: 1 - 2 0 i.e. (1 2)
Assuming population variances are equal, we would have to calculate pooled-variance t-Test
Sp^2= (n1-1)S1^2+(n2-1)S2^2/(n1-1)+(n2-1)
= (10-1)*6.27^2+(10-1)*7.26^2/9+9
= 354.1+474.4/18
= 46.027
tSTAT=(X1-X2)-(µ1-µ2)/Sp^2(1/n1+1/n2)
=(20.7-23.6)-0/46.027(1/10+1/10)
=-2.9/3.03
=-0.9558
tCRIT is +/-2.1 and hence cannot reject the null hypothesis
Cohen’s d=(M1-M2)/Sp
Cohen’s d=23.6-20.7/6.78=0.427 (Medium)
Steps to conduct independent samples t-test in SPSS: -
1) Analyze>Compare Means>Independent Samples T Test
2) Insert the Test Variable and Grouping Variable
3) Click OK
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