If the concentration of vitamin C in oranges is related to the length
of time it has been removed from the tree, then oranges freshly
picked will have higher levels of the vitamin.
The experimental groups for this experiment should be varying times after
the fruit is picked. It is important that these groups be selected carefully to show
an adequate spread of results. If the vitamin C levels are measured in oranges
at intervals of 6 hours, 12 hours, and 24 hours, the levels of vitamin might not
differ enough to notice. Similarly, experimental groups divided into extreme
high and low quantities will not show the detail needed to analyze the effects.
If experimental groups in the orange/vitamin C experiment are measured at
1 day, 4 weeks, 8 weeks, and 16 weeks, the levels of vitamin C might be drastically
different, but, without multiple gradients of the independent variable,
there is no subtle data to determine critical levels or provide insight as to why
those changes might have occurred. Therefore, it is important that you use
background research to study the variables so that your groups can be set up
appropriately.
It is also possible that experimental groups cannot be predetermined.
Sometimes, it is only after the data are collected that data can be grouped for
analysis. For the river otter experiment shown in Figure 2.1, the data could
be categorized into groups based on the range of temperatures that were
actually observed on data collection days. The independent variable of this
experiment is the change in water temperature and the dependent variable is
the frequency of river-otter behaviors that the researcher categorizes as active
or nonactive when making observations.