What is the difference between statistical significance and practical significance?

is typically defined when the chance of falsely rejecting the null hypothesis is less than 0.05. To put it another way statistical significance occurs when there is less than a 5% chance that the measured result between groups is truly present and not just due to random effects.

Practical significance as the name implies denotes the real world importance of any statistically significant result. In health research, this is referred to a the minimal clinical important difference (MCID). For example, it may be all well and good that a medication makes you one millimetre taller but practically no one would care. As the sample size of a study increases the chance of finding a statistically significant but practically insignificant result increases.

Another consideration is the fact that the 5% cut off commonly used to determine statistical significance is entirely arbitrary. If a life saving drug is found to save lives but the chance that the results of the study are due to random chance are 6% and not below the 5% cut-off, it is very unlikely that the results will not have practical significance. Results such as that will not be dismissed on the basis of one percentage point.