An investigator wishes to examine the ability of a new drug treatment administered as eye drops to reduce bilaterally increased intraocular pressure (IOP).

The investigator randomizes treatment to eyes of patients with bilaterally increased intraocular pressure such that 50 patients are assigned to active treatment for their right eye and 50 patients are assigned active treatment for their left eye. The drug has been shown to affect only the treated eye. The patient's other eye is assigned placebo therapy. After one month of treatment the investigator measures the intraocular pressures in both eyes. The investigator then calculates the after treatment difference between the measurements of IOP in the drug treated eye minus the placebo treated eye in mmHg.

Let us use the flowchart of statistics to address two basic questions of analysis.

1. Estimation: What measurement should be used to summarize the data which have been found in this investigation?

2. Inference: How can we use the study data to investigate whether there is a real difference in intraocular pressure between drug treated and placebo treated eyes for individuals with bilaterally increased intraocular pressure in the population from which the sample was selected?