Selection of the response variates to be measured

The Plan must include a step in which we decide what variates we will measure on each unit to be selected in the sample. Response variates, corresponding as much as possible to those used to define attributes of interest in the target population, must be clearly defined.

Michelson could not measure the speed of light on a unit directly with his apparatus. Instead, for each determination, he measured the following response variates to calculate the speed of light.

1.

the displacement d of the image in the slit. This was measured on each unit.

2.

the radius r, the distance between the cross-hairs of the slit and the front face of the rotating mirror. This value was not always determined for units measured in the same time period but was measured each morning or evening when units were sampled.

3.

the number of beats B per second between the electric Vt₂ fork and the standard Vt₃. This variate was determined once for each set of 10 determinations of d.

4.

the temperature T measured once for each set of 10 determinations of d.

The values of the response variates were combined with several constants according to the formulae (3) and (4) ([39] page 133) to produce a value for the speed of light in air at temperature T.