“Statistical significance” (as contrasted with biological significance) means statistical analysis has revealed an effect unlikely to have occurred by chance alone. The level of significance refers to the degree to which the result could be explained by chance. At the 0.05 (5%) level the result could have occurred by chance 1 time in 20; at the 0.01 (1%) level the result could have occurred by chance only 1 time in 100. Any effect observed in a study or experiment carries with it some degree of uncertainty, or imprecision, because of randomness and variability in most biological phenomena. Statistical techniques evaluate an observed effect in view of its precision to determine with what probability it might have arisen by chance (the level of significance). Values with a low probability of occurring by chance are called “statistically significant” and are thought to represent a real effect.