Lab 5
Graded 3
Please complete the following attached exercise.
Do all needed calculations by PCSAS and
SPSS. Calculations need to be done by hand also.
The following procedure produces the necessary univariate descriptive
statistics (measures of central tendency). Due (by beginning class
2/5).
Hand in the sas program and output. I just need output for SPSS.
--General Form:
Proc Univariate data=data set name;
Var
variable-list;
Run;
--For SPSS, you can still go to
Statistical Guide: A distribution of scores is said to be skewed to the right (i.e., positively skewed) when scores are concentrated near the low end of a distribution with a small number off to the right (toward the high end). The mean for a distribution that is skewed to the right is pulled up by the skew. A distribution of scores is said to be skewed to the left (i.e., negatively skewed) when scores are concentrated near the high end of a distribution with a small number off to the left (toward the low end). The mean for a distribution that is skewed to the left is pulled down by the skew. When a distribution is severely skewed, the mean may be an unrepresentative average. The median, in contrast, is unaffected by skewness.
Background Notes: Among other things, researchers asked a sample of college men taking an introductory psychology course how many sexual partners they have had in their lifetime. All of the men were unmarried and reported being heterosexual. Their ages ranged from 17 to 28, with a mean age of 19.
Making Predictions: Before examining the data below, predict the results you will obtain. (When scientists make predictions, they are hypothesizing.) Note that your predictions are not right or wrong. Rather, they represent your best guess as to the outcomes you will obtain. After you perform the calculations, you will be able to determine whether the data support your predictions.
1. Predict whether the distribution of the number of sexual partners is skewed and, if
so, whether it is skewed to the right or skewed to the left.
A. Yes, it is skewed to the right, with most men having few partners but a
small number having many partners.
B. Yes, it is skewed to the left, with most mean having many partners but a
small number having few partners.
C. No, it is not skewed; it is a symmetrical distribution.
2. If your prediction to question 1 is correct, which of the following is true?
A. The mean number of partners is higher than the median number.
B. The median number of partners is higher than the mean number.
C. The mean number and median number of partners are about the same.
Data: Table 1 shows the frequency distribution.
Table 1 Number
of lifetime sexual partners
Number of partners
Frequency (f)
88 1
25 1
21 1
20 1
15 1
12 1
11 1
10 1
7 1
6 1
5 1
4 4
3 7
2 6
1 13
0
24
Calculations:
3. What is the mean number of sexual partners? (If you are using a calculator, you
can save time by multiplying each number of partners by the associated
frequency and summing the products to get the sum of scores. Dividing this by
65 will yield the answer.)
4. What is the median number of sexual partners? (Ask your instructor whether he
or she wants you to determine the approximate median by counting up 50% of the
cases or to calculate the precise median determined with a formula that
interpolates within the interval of interest.) I want crude and interpolate medians
Checking Your Predictions:
5. Does your analysis confirm your prediction in question 1?
6. Does your analysis confirm your prediction in question 2?
Questions for Discussion:
7. If you were writing a report on the data in this exercise and were allowed to report
only one average, would you choose the mean or the median? Explain.
8. What is the value of the mode of the distribution in this exercise? Would it be a
good choice as an average for this distribution? Explain.
9. Sometimes subjects distort their replies on questionnaires to give socially
desirable answers. Do you think this might have happened in this study? If so, do
you think the men might have over reported or underreported the number of
lifetime sexual partners they had?