Stat 301
HW 3 Answers
Fall 2015
2 pts each part except problem 3.
1) Oiled and unoiled birds
a) pooled sd = 6.25
Notes: You need to start with the variances to get the pooled estimate. An answer of 39 is partial
credit: you’re reporting the pooled variance.
b) 94
Note: Computed as 96 obs – 2 means, or 36 df for unoiled and 60 df for oiled gives 96 for pooled
2) Bulimia.
Note: We presumed you would use JMP for all calculations, since you have the data.
a) Yes, assuming equal variances is appropriate. The ratio of sd’s (5.29 / 4.92 = 1.1) is less than 2.
b) pooled sd = 5.13
Note: Given as “Root Mean Squared Error” in the JMP / Analzye / Fit Y by X / ANOVA/pooled T
results. (Where to find this number in the JMP output in Lab 4 instructions, step 11 or in the
summary at the end).
c) 95% CI is (-7.95, 0.601) (or negative of this)
d) p > 0.05, since the 95% ci includes 0
e) p = 0.0886 (or rounded to 0.089)
3) Answers vary considerably. For full credit, it contains:
a) an estimate of the difference
b) how precise is that estimate
Note: a and b could be answered with an estimate and se or by a confidence interval
c) two-sided p-value (or inferred p-value from the CI)
d) a statement about what these data tell you about potential price fixing. E.g., are they consistent
with the price fixing hypothesis? Alternatively, how strong is the evidence for price fixing?
It would be very helpful to include a statement of how you computed the p-value and CI: e.g., twosample T test assuming equal variances.
Note: Other things that could be very useful to include because they help interpret the differences:
a) Average prices in the two regions.
b) pooled sd
c) a graph showing the data or showing the two region averages and their precision (+/- 1 se bars
or confidence intervals)