Fig. 3. When variability of individual PPRs increases, mean PPRs also increase. *A*, The individual traces at the*top* are representative samples from a single cell before and after Cd^{2+} (30 μm) application. The mean PPR increased from 0.78 to 1.16 (*dotted lines*) as in the graph of the complete set of data from this cell. One*dot* represents one individual PPR. Individual PPRs range from very low to very high. *B*, Trace groups with larger mean PPRs have higher variability of individual PPRs. SD values of PPR (SD_{PPR}) within a trace group were plotted against the mean PPR of the trace group. One *dot* represents one trace group. The variability is fairly constant below a PPR of ∼0.85 in contrast to the variability when the PPR > 1.0 (the PPF region), in which variability increases proportionally with PPR. In the PPR region between 0.85 and 1.0 there is an intermediate pattern of variability. *C*, When mean PPR increased, the largest individual PPRs increased, and the smallest PPRs decreased. The means of the three largest PPRs (*open circles*) and of the three smallest PPRs (*filled circles*) in a trace group were calculated from trace groups that showed a mean PPR increase after the treatments. Data were divided into three groups according to the PPR after treatment (PPR_{treat}). In the group of PPR_{treat} < 0.85 (*n* = 15 trace groups), the largest and the smallest PPRs did not change (paired*t* tests, *p* > 0.1). In the other two groups (*n* = 23 trace groups each), however, the largest PPRs increased and the smallest PPRs decreased when mean PPR increased. Data from all treatments were pooled because they showed similar patterns individually. The *left dot* in each pair represents the control value, and the *right dot*represents the value after treatment. The error bars are SEM. The numbers of trace groups treated by reduced stimulation, Cd^{2+}, or baclofen, respectively, included the following: for PPR < 0.85: 3, 4, and 8; for PPR between 0.85 and 1: 6, 9, and 8; for PPR > 1: 11, 8, and 4. *Paired*t* test, *p* < 0.01. *D*, The largest and smallest PPRs are related inversely in groups showing mean PPF. Each *symbol* represents the mean of the three largest PPRs of a trace group plotted against the mean of three smallest PPRs of the given trace group. Data are from trace groups for which the mean PPRs were >1.0. The *solid lines* in*D–F* are the *y* = 1/*x*curves, which describe a perfect inverse relationship.*E*, The same plot as in *D* made for groups with mean PPRs below 0.85. *F*, The same plot as in*D* and *E* for simulated data. *Filled circles* are trace groups with a CV of A1 > 0.3;*open circles* are trace groups with a CV of A1 < 0.3.