A study described in Data A.1 on page 136 found that mice exposed to dim light at night gained substantially more weight than mice that had complete darkness at night, despite the fact that calorie intake and activity levels were the same for the two groups. Was it the light that caused the mice to gain weight or was it the fact that they ate a large percentage of their calories when mice would normally be sleeping? The same researchers conducted a second experiment to answer this question. The second experiment matched the first in every respect except a new variable was added: half of the mice could eat whenever they wanted while the other half could only eat at night (the normal time mice eat). The mice in dim light that could eat whenever they wanted grew fatter, matching the results in the first experiment almost exactly. However, the mice in dim light that could only eat at night stayed as slim and healthy as the mice in darkness.

(a) In the new experiment, two variables are manipulated. What are they? Are they categorical or quantitative?

(b) A question is asked in the description above. Answer it, using the information that follows it.  

Data A.1 on page 136

Numerous studies have shown that exposure to light at night is harmful to human health. A recent study in mice shows that even low-level light at night can interfere with normal eating and sleeping cycles. Furthermore, the study finds that food is especially fattening if consumed at the wrong time of day, at least in mice. In the study, 27 mice were randomly split into three groups. One group was on a normal light€“dark cycle (LD), one group had bright light on all the time (LL), and one group had light during the day and dim light at night (DM). The dim light was equivalent to having a television set on in a room. The mice in darkness ate most of their food during their active (nighttime) period, matching the behavior of mice in the wild. The mice in both dim light and bright light, however, consumed more than half of their food during the well-lit rest period, when most mice are sleeping. Although the data collected show that the three groups of mice ate approximately the same amount of food and had the same levels of physical activity, the mice exposed to light at night gained substantially more weight, ate a greater percent of calories during the day, and were more likely to be classified as glucose intolerant at the end of the study. An absence of darkness seems to be associated with fatter mice.