ANS: A controlled experiment (10 points)
ANS: The subjects were not randomly assigned to treatment
or control. The first 20 arrivals are likely to be early risers who
don't need or react to caffeine differently than do the late
arrivals. (15 points)
ANS: The median, because the half-life is the time until half the uranium decays. In the data, it is the time value which half the lifetimes are greater than and half are smaller than: namely, the median of the lifetimes.
ANS: No. The location of the histograms is dropping, from around 44 in batch 1 to around 39.5 in batch 5. (10 points)
ANS: Figure 2a would result. (10 points)
ANS: An observational study, since treatments (stretching) were not assigned to sampling units (runners). (10 points)
ANS: No, since this is not a controlled experiment, we can only conclude there is association between stretching and injuries. (10 points)
ANS: (i) Those who stretch may do so because they have pre-existing injuries, or (ii) runners who stretch may be more competitive, resulting in more injuries, or some other explanation. (10 points)
ANS: Not true. E.g.: 6 data values, -4, -2, -1, 1, 3, 3. Median=mean=0.