Figure 3. Box chart representing Knoop microhardness data for enamel from 2 maxillary incisors each from MMP-20 null mice (KO) and wild-type mice (WT). The Y-axis shows Knoop hardness numbers (KHN). The boxes outline the 25–75% data range, whiskers represent the standard deviation, squares in the boxes correspond to the mean hardness value, crosses (X) delimit the 99% data range, and hyphens (-) represent minimum and maximum hardness values. Average hardness for wild-type enamel (WTenamel) is 327 KHN₁₀ (SD ± 70) and 207 KHN₁₀ (SD ± 63) for MMP-20 null enamel (KOenamel). Student’s t test analysis demonstrated that the two means were significantly different (p = 0.005).