Statistical Analyses of Distance Measurements. The determined initial contact position D C , equilibrium separation D eq , position of maximum adhesion D Adh , and thickness in the absence of ligand D T are averages of 7À20 measurements from 2À3 independent experiments. Standard deviations are the pooled standard deviations from separate sets of measurements. Averages were compared using the student's t test, and p-values <0.05 indicate a statistically significant difference, at the 95% confidence level. The standard deviation of the difference between pairs of values was determined by propagation of errors.