about, and you randomize to control for the effect of those you don’t know about. In this case, then,
you randomize to control for any unknown systematic differences between the plots that might
influence sweetness. An example might be that the plots on the northern end of the rows (plots 1 and
6) have naturally richer soil than those plots on the south side.
The idea is to get plots that are most similar in order to run the experiment. One possibility would be
to match the plots the following way: close to the river north (6 and 7); close to the river south (9 and
10); away from the river north (1 and 2); and away from the river south (4 and 5). This pairing
controls for both the effects of the river and possible north–south differences that might affect
sweetness. Within each pair, you would randomly select one plot to plant one variety of strawberry,
planting the other variety in the other plot.
This arrangement leaves plots 3 and 8 unassigned. One possibility is simply to leave them empty.
Another possibility is to randomly assign each of them to one of the pairs they adjoin. That is, plot 3
could be randomly assigned to join either plot 2 or plot 4. Similarly, plot 8 would join either plot 7
or plot 9.
- The study could have been double-blind. The question indicates that the subjects did not know which
treatment they were receiving. If the psychologists did not know which therapy the subjects had
received before being evaluated, then the basic requirement of a double-blind study was met: neither
the subjects nor the evaluators who come in contact with them are aware of who is in the treatment
and who is in the control group.
If the study wasn’t double-blind, it would be because the psychologists were aware of which
subjects had which therapy. In this case, the attitudes of the psychologists toward the different
therapies might influence their evaluations—probably because they might read more improvement
into a therapy of which they approve. - Group A favors a dress code, group B does not. Both groups are hoping to bias the response in favor
of their position by the way they have worded the question. - You probably want to block by community since it is felt that economic status influences attitudes
toward advertising. That is, you will have three blocks: Upper Middle, Middle, and Lower Middle.
Within each, you have four billboards. Randomly select two of the billboards within each block to
receive the Type I ads, and put the Type II ads on the other two. After a few weeks, compare the
differences in reaction to each type of advertising within each block. - With only 3000 of 100,000 surveys returned, voluntary response bias is most likely operating. That
is, the 3000 women represented those who felt strongly enough (negatively) about men and were the
most likely to respond. We have no way of knowing if the 3% who returned the survey were
representative of the 100,000 who received it, but they most likely were not. - Assign each of the 26 women a two-digit number, say 01, 02, ..., 26. Then enter the table at a
random location and note two-digit numbers. Ignore numbers outside of the 01–26 range. The first
number chosen assigns the corresponding woman to the first group, the second to the second group,
etc. until all 26 have been assigned. This method roughly equalizes the numbers in the group (not
quite because 4 doesn’t go evenly into 26), but does not assign them independently.
If you wanted to assign the women independently, you would consider only the digits 1, 2, 3, or 4,
which correspond to the four groups. As one of the women steps forward, one of the random digits is
identified, and that woman goes into the group that corresponds to the chosen number. Proceed in this
fashion until all 26 women are assigned a group. This procedure yields independent assignments to
groups, but the groups most likely will be somewhat unequal in size. In fact, with only 26 women,