of industrial explosion and were under age 40 at the time of accident had heightened concentrations of dioxin in their blood and that each tenfold increase in dioxin level was associated with a doubling of the risk of breast cancer.
a) In any experiments, it is required to select sample randomly. If the subjects were not assigned to the diets randomly instead of letting people pick what they wanted to eat, than there may be possibility of adding another factor (in this case subject’s choice) introduced to the experiments. Thus, it will be difficult to say which factor influenced blood pressure more effectively, subject’s choice what they wanted to eat or diet type.
Randomization allows us to equalize the effects of unknown or uncontrollable sources of variation. It does not eliminate the effects of these sources, but it spreads them out across the treatment levels so that we can see past them.
c) The researchers need the control group because than they will be able to compare the blood pressure of groups based on different diet only. If the DASH diet group’s mean blood pressure was lower at the end of the experiment than at the beginning that would only mean that DASH diet lowered the blood pressure. This will not mean how much effective DASH diet is compared to other diets. Further, in an experiment it is required to control sources of variation other than the factors we are testing by making conditions as similar as possible for all treatment groups.
d) We would need additional information standard error of difference (or sample standard deviations and sizes for DASH diet group and control group) in order to decide whether an average reduction in blood pressure of 6.7 points was statistically