Example 3: experiment
a. an experiment imposes treatments to see the effect of the treatments on some response. suppose that an observational study indicated that a certain type of tree did not have as much
termite damage as other trees. researchers wondered if resin from the tree was toxic to termites. they decided to do an experiment where they exposed some termites to the resin and
others to plain water and recorded whether the termites survived. the explanatory variable (treatment variable) is the exposure type (resin, plain water), and the response variable is
whether or not the termites survived. we know this is an experiment because the researchers imposed a treatment (exposure type) on the subjects (termites).
is the following an observational study or an experiment? why? if it is an experiment, identify the treatment variable and the response variable. if it is an observational study, identify the
population of interest.
a study was done to answer the question, "what is the effect of different durations of light and dark on the growth of radish seedlings? " three similar growth chambers (plastic bags)
were created in which 30 seeds randomly chosen from a package were placed in each chamber. one chamber was randomly selected and placed in 24 hours of light, another for 12 hours
of light and 12 hours of darkness, and a third for 24 hours of darkness. after three days, researchers measured and recorded the lengths of radish seedlings for the germinating seeds.
b. in an experiment, random assignment of subjects to treatments is done to create comparable treatment groups. for example, a university biologist wants to compare the effects of two
weed killers on pansies. she chooses 24 plants. if she applies weed killer a to the 12 healthiest plants and b to the remaining 12 plants, she will not know which plants died due to the
type of weed killer used and which plants subjected to weed killer b were already on their last legs. randomly selecting 12 plants to receive weed killer a and then assigning the rest to
b would ensure that the plants in each group are fairly similar.
how might the biologist go about randomly assigning 12 plants from the 24 candidates to receive weed killer a? could she be sure to get exactly 12 plants assigned to weed killer a and
12 plants to weed killer b by tossing a fair coin for each plant and assigning "heads up" plants to weed killer a and "tails up" to weed killer b? if not, suggest a method that you would use.
c. write or say to your neighbor two things that are important about experiments.