Definition of "percentile" I'm now reading a note on Biostatistics written by PMT Education, and notice the following sentences in Section 2.7:

A baby born at the 50th percentile for mass is heavier than 50% of babies.
  A baby born at the 25th percentile for mass is heavier than 75% of babies.
  A baby born at the 75th percentile for mass is heavier than 25% of babies. 

But as I know, a baby born at the 25th percentile for mass should be heavier than 25% of babies. Is there a special definition of "percentile" in this field, or I misunderstand the sentences as a non-native speaker? 
 A: While the definition of percentiles given by Stephen Kolassa is technically correct in statistical theory (the best kind of correct?), this is an issue where there is a lot of variation in practice --- some people refer to percentiles with the highest percentile as the maximum, but others flip it over so that the highest percentile is the minimum.  In the latter case people will sometimes talk about someone being in the 5th percentile when they are in the top five percent, rather than the bottom five percent.  Sometimes they will say this explicitly (e.g., John Smith is in the top 5th percentile for shot-put distance), but sometimes they won't specify this clearly.  For this reason, it is always important to clarify with the reader/speaker which way around they are defining the percentiles.  (In the absence of any specification to the contrary, they should really use the standard statistical definition.)
Also, I disagree with Stephen on one point.  I doubt this is a typographical error.  More likely, the writer of the document is simply speaking of percentiles in the second sense I have described, which while not technically correct, is nonetheless quite common.  I don't really regard this as an "error" so much as a non-standard use of the term, which is excusable if it is explained.  Here is an example of the reversed use of "percentiles" in an article on income levels in the Wall Street Journal.  (Most instances of reversal of the percentages occur in the context of discussions of wealth/income levels.  Though it is much less common than the technically correct usage, it occurs commonly enough that you need to be careful to check the meaning.)  Here is a follow-up question where I seek examples of this reversed practice.
A: This is just a typo in the document. Your understanding of percentiles is correct.
