Regarding observational studies and designed experiments: a. Describe each type of statistical study. b. With respect to possible conclusions, what important difference exists between these two types of statistical studies?

Step 1
(a)
Statistical studies can be broadly classified into two types, depending upon the mode of data collection- observational studies and designed experiments.
Observational study:
The investigators may simply observe a predefined set of individuals and collect the data from the observations, without influencing any factors involved. This is an “observational study”.
Designed experiment:
The investigator may design and perform their own experiments by controlling and manipulating factors of their choice and collect the resulting data. This is the “designed experiment”.
Step 2
(b)
In designed experiments, the researcher has the ability to control or manipulate the factors. As a result, the researcher can determine if the factor is ‘causing’ the outcome. Thus, designed experiments can help establish causation.
However, in observational studies, the researcher may simply observe the happenings, without any power to control or manipulate the factors. Thus, no causation can be determined in observational studies.
Thus, the important difference between the two types of statistical studies, with respect to possible conclusions is that designed experiments can help establish causation, whereas observational studies cannot establish causation.