Many temperate-zone animal species exhibit physiological and morphological changes when the hours of daylight begin to decrease during autumn months. A study was conducted to investigate the “short day” traits of collared lemmings (The Journal of Experimental Zoology, Sept. 1993). A total of 124 lemmings were bred in a colony maintained with a photoperiod of 22 hours of light per day. At weaning (19 days of age), the lemmings were weighed and randomly assigned to live under one of two photoperiods: 16 hours or less of light per day, more than 16 hours of light per day. (Each group was assigned the same number of males and females.) After 10 weeks, the lemmings were weighed again. The response variable of interest was the gain in body weight (measured in grams) over the 10-week experimental period. The researchers analyzed the data using an ANOVA for a 2 × 2 factorial design, where the two factors are photoperiod (at two levels) and gender (at two levels).

a. Construct an ANOVA table for the experiment, listing the sources of variation and associated degrees of freedom.

b. Give the models that will enable the researchers to test for photoperiod by gender interaction.

c. The F test for interaction was not significant. Interpret this result practically.

d. The p-values for testing for photoperiod and gender main effects were both smaller than .001. Interpret these results practically.