Question

The males of stalk-eyed flies have long eye stalks. The females sometimes use the length of these eye stalks to choose mates. Is the male's eye-stalk length affected by the quality of its diet? An experiment was carried out in which two groups of male "stalkies" were reared on different foods. One group was fed corn (considered a high quality food), and the other was fed cotton wool ( a food of substantially lower quality). Each male was raised singly and so represents an independent sampling unit. The eye spans (the distance between the eyes) were recorded in millimeters. The raw data is as follows:

Corn diet: 2.15, 2.14, 2.13, 2.13, 2.12, 2.11, 2.10, 2.08, 2.08, 2.08, 2.04, 2.05, 2.03, 2.02, 2.01, 2.00, 1.99, 1.96, 1.95, 1.93, 1.89

Cotton diet: 2.12, 2.07, 2.01, 1.93, 1.77, 1.68, 1.64, 1.61, 1.59, 1.58, 1.56, 1.55, 1.54, 1.49, 1.45, 1.43, 1.39, 1.34, 1.33, 1.29, 1.26, 1.24, 1.11, 1.05

These data can be summarazied as follows, where the corn-fed flies represent treatment group 1 and the cotton-fed flies represent treatment group 2.

	Mean (mm)	Variance (mm2)	Sample size, n
Corn diet (group 1)	2.05	0.00558	21
Cotton diet (group 2)	1.54	0.0812	24

a) What is the best test to use for comparing the means of the two groups? why?

b) Carry out the test identified in part (a), using alpha=0.01

c) give a 99% confidence interval for the difference in mean eye span between corn-fed and cotton-fed stalk eyed flies

Answer 1