7–16. What’s the design? Read each brief report of...

7–16. What’s the design? Read each brief report of statistical research, and identify a) whether it was an observational study or an experiment. If it was an observational study, identify (if possible) b) whether it was retrospective or prospective. c) the subjects studied and how they were selected. d) the parameter of interest. e) the nature and scope of the conclusion the study can reach. If it was an experiment, identify (if possible) b) the subjects studied. c) the factor(s) in the experiment and the number of levels for each. d) the number of treatments. e) the response variable measured. f) the design (completely randomized, blocked, or matched). g) whether it was blind (or double-blind). h) the nature and scope of the conclusion the experiment can reach. Vitamin C doping Some people who race greyhounds give the dogs large doses of vitamin C in the belief that the dogs will run faster. Investigators at the University of Florida tried three different diets in random order on each of five racing greyhounds. They were surprised to find that when the dogs ate high amounts of vitamin C they ran more slowly. (Science News, July 20, 2002)

7–16. What’s the design? Read each brief report of statistical research, and identify
a) whether it was an observational study or an experiment. If it was an observational study, identify (if possible)
b) whether it was retrospective or prospective.
c) the subjects studied and how they were selected.
d) the parameter of interest.
e) the nature and scope of the conclusion the study can reach.
If it was an experiment, identify (if possible)
b) the subjects studied.
c) the factor(s) in the experiment and the number of
levels for each.
d) the number of treatments.
e) the response variable measured.
f) the design (completely randomized, blocked, or matched).
g) whether it was blind (or double-blind).
h) the nature and scope of the conclusion the experiment can reach.
Vitamin C doping Some people who race greyhounds give the dogs large doses of vitamin C in the belief that the dogs will run faster. Investigators at the University of Florida tried three different diets in random order on each of five racing greyhounds. They were surprised to find that when the dogs ate high amounts of vitamin C they ran more slowly. (Science News, July 20, 2002)

Key Concepts

Scope of Conclusion

The scope of conclusion refers to how broadly the findings of a study can be generalized. In observational studies, conclusions are generally confined to observed associations and may be limited by potential confounding factors, whereas well-designed experiments can support broader causal inferences, assuming the study has controlled for other influences.

Blinding

Blinding involves keeping study participants, and sometimes researchers, unaware of the treatment assignments in order to prevent bias in the collection and interpretation of data. Single-blind studies keep subjects unaware, while double-blind studies ensure that neither the subjects nor the researchers know who is receiving which treatment.

Randomization

Randomization is the method of assigning subjects to different groups or treatment conditions purely by chance, thereby minimizing bias and ensuring that any differences observed are due to the treatments rather than other factors. It is a cornerstone of valid experimental design.

Factors, Levels, and Treatments

In experiments, factors are the independent variables that are purposefully manipulated, and each factor can have multiple levels (different conditions or values). The combination of these levels across factors constitutes the various treatments administered to subjects, allowing researchers to examine and compare the effects of different experimental conditions.

Experimental Design Types

Experimental design types, including completely randomized, blocked, and matched designs, refer to methods of assigning subjects to treatment groups in ways that control for external or confounding variables. These designs help to increase the precision and validity of causal conclusions drawn from the experiment.

Subjects and Sampling

This concept focuses on the selection and definition of study participants or experimental units. Robust study design requires clearly identifying the population of interest and employing appropriate sampling methods, whether in observational studies or experiments, to ensure that the results are representative and reduce bias.

Retrospective and Prospective Studies

These terms characterize observational studies based on the timing of data collection relative to the occurrence of events. Retrospective studies analyze past records and events, while prospective studies follow subjects forward in time to observe outcomes as they occur, each approach having distinct advantages and limitations in studying associations.

Experimental Study

An experimental study is designed to assess the effect of one or more factors by deliberately manipulating them and observing the outcomes on a response variable. This design typically includes features such as randomization and control groups, which help establish causality between the factors and the outcomes.

Observational Study

An observational study involves collecting data without intervening in or altering the conditions under which the data are obtained. Researchers simply observe and record information about subjects or events, which is useful for identifying associations but does not generally confirm causal relationships.

Parameter of Interest

The parameter of interest is the specific quantity, such as a mean, proportion, or difference between groups, that the study is designed to estimate or test. It is the focal point of the statistical analysis and inference, guiding the collection and interpretation of data.

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