Surgeon and institution volume were defined as low (first quintile), medium (second, third, or fourth quintile), and high (fifth quintile). Stratification by institution volume and then by surgeon volume was performed to analyze the primary endpoint: in-hospital mortality. - https://pubmed.ncbi.nlm.nih.gov/21144692/
What is a Quantile?
Quintile | definition of quintile by Medical dictionarymedical-dictionary.thefreedictionary.com › ... - any of the four values that divide the range of a probability distribution distribution into five parts of equal probability; the first, second, third, and fourth quintiles - https://www.google.de/search?hl=de&ei=rkydX4y2A420gQbun6PICA...
The word “quantile” comes from the word quantity. In simple terms, a quantile is where a sample is divided into equal-sized, adjacent, subgroups (that’s why it’s sometimes called a “fractile“). It can also refer to dividing a probability distribution into areas of equal probability. - https://www.statisticshowto.com/quantile-definition-find-eas...
A quintile is a 1/5th (20 percent) portion of the whole. In statistics, it’s a population or sample divided into five equal groups, according to values of a particular variable. It can also be used to refer to the cut-of points (quartiles) for these portions.

0 – 20% is the first quintile (also called the bottom quintile),
From 20 to 40% is the second quintile,
From 40 to 60% is the third quintile,
From 60 to 80% is the fourth quintile,
From 80 to 100% is the fifth quintile (also called the top quintile).