Why Experiment?

Why Experiment?

1. Experiments help us answer questions

   • Experiments allow us to set up a direct comparison between the treatments of interest.
   • We can design experiments to minimize any bias in the comparison.
   • We can design experiments so that the error in the comparison is small.
   • We may make inferences about causation.

2. Mosteller and Tukey (1977) list three concepts associated with causation and state that two or three are needed to support a causal relationship: • Consistency • Responsiveness • Mechanism.

   • The drawback of observational studies is that the grouping into “treat- ments” is not under the control of the experimenter and its mechanism is usually unknown.

3. A good experimental design must • Avoid systematic error • Be precise • Allow estimation of error • Have broad validity.

   • Comparative experiments estimate differences in response between treat- ments. If our experiment has a systematic error, then our comparisons will be biased, no matter how precise our measurements are or how many experimental units we use.
   • Experiments must be designed so that we have an estimate of the size of random error.
   • The conclusions we draw from an experiment are applicable to the experimental units we used in the experiment

4. Terms & Concepts

   • Treatments are the different procedures we want to compare.
     • Experimental units are the things to which we apply the treatments.
       • Responses are outcomes that we observe after applying a treatment to an experimental unit.
         • Randomization is the use of a known, understood probabilistic mechanism for the assignment of treatments to units.
           • Experimental Error is the random variation present in all experimental results.
             • Measurement units (or response units) are the actual objects on which the response is measured.
               • Blinding occurs when the evaluators of a response do not know which treatment was given to which unit.
                 • Control has several different uses in design.
                   • Placebo is a null treatment that is used when the act of applying a treatment— any treatment—has an effect.
                     • Factors combine to form treatments.
                       • Confounding occurs when the effect of one factor or treatment cannot be distinguished from that of another factor or treatment.