Techniques for choosing reference points from which to measure eruption rates of rat incisors.

Measurements of eruption rates were compared with a computer analysis of measurements of eruption rates from stable and unstable reference points. In the computer analysis, when the measurements were made from unstable reference points, there were correlations between eruption rates and changes in tooth length and between unimpeded and impeded eruption rates. These correlations were not seen in measurements made from stable reference points. The measurements from unstable reference points had greater variances. These correlations were found in measurements made from the gingival margin of the unimpeded incisor but not in measurements from the margin of the impeded incisor. The measurements from the impeded incisor had a lower variance. The ability was demonstrated of these correlations and variance ratios to detect instability of reference points.