PERCENTILE AND ITS USES IN HOSPITALS, THE STOCK MARKET, AND SCHOOLS
You may have heard the term percentile in school or doctor’s offices because it is almost always used when considering statistics.
There is no standard definition of percentile yet all definitions yield similar results when the number of observations is very large. Percentile tells us if our measurements or scores are average or not.
What percentile is used for
In statistics, a percentile (or centile) is the value of a variable below which a certain percent of observations fall.
The 25th percentile is also known as the first quartile (Q1), the 50th percentile as the median or second quartile (Q2), and the 75th percentile as the third quartile (Q3).
The Term Percentile As Used by Doctors
I researched the term and found this question posted by a very concerned dad:
Fetal weight under 50th percentile. What does it mean?
My wife and I will be having our first baby. I live away from my wife. Today she called me and informed me that the doctors told her, that fetus's weight is under 50th percentile, and the legs of the fetus were about 7 cm. What exactly does this mean? I am really worried. I am not sure what this means. Can anyone please shed their expert light on this topic and what I should be doing?
These were some of the answers:
Response 1: Being 'under' the 50th percentile means that your baby is pretty close to 'average' (technically: 'Statistical Median') weight.
Response 2: All that less than 50th percentile means is that she is bigger than less than half of the fetuses her age. Nothing to be concerned about. If she were in the 75th percentile, that means she's bigger than 75 out of those 100 fetuses. Perfectly normal either way.
Response 3: "Well it means that your baby so far is smaller than average, without knowing how far along she is i cant be much more help than that im sorry."
Response 4: "Small ish baby. 50th percentile = 50 babies out of 100 would be bigger, and 50 smaller."
Conclusion: My heart aches for this poor father-to-be! One cannot imagine how all this conflicting information made him feel. People do not seem to understand percentile so maybe it would help if doctors, instead, said "below average weight for fetus of the same age." It would probably be more clear, especially if patients or clients are not English-proficient and using an interpreter. In Response 1, the writer seemed to interpret "under" as under-the-label-of when referring to percentile. If you are the patient and do not understand what your practitioner is saying, ask him/her to explain.
From Help Guide.org:
A child’s “BMI-for-age” shows how his or her BMI compares with kids of the same age. A child between the 85th and 95th percentile on the growth chart is considered at risk of overweight. A child at the 95th percentile or above considered overweight.
Weight and height percentiles are determined by growth charts and body mass index charts to compare a child's measurements with those of other children in the same age group. By doing this, doctors try to track a child's growth over time and monitor how a child is growing in relation to other children. There are different charts for boys and girls because their growth rates and patterns differ. For both boys and girls there are two sets of charts: one for infants 0 to 36 months old and another for children 2 to 20 years old.
Creating a chart that reflects how many children have certain measurements and calculating percentiles is not the same thing. Percentiles measure deviation from the median or average measurements (or scores). Percentiles go up or down, away from the average.
Most doctors seem to interpret the Weight/Height Chart as follows: the 50% line is the median, the average. When the chart was created, of all the children measured, most of them had certain measurements and therefore these measurements are presented as the 50th percentile, median, or average. When comparing a real child to the chart, if a child’s weight and height are within those of the 50th percentile, he/she is average; the child is in the 50th percentile.
From Johns Hopkins Children’s Center:
Failure to Thrive Children are diagnosed with failure to thrive when their weight or rate of weight gain is significantly below that of other children of similar age and gender. Infants or children that fail to thrive seem to be dramatically smaller or shorter than other children the same age. Teenagers may have short stature or appear to lack the usual changes that occur at puberty. However, there is a wide variation in what is considered normal growth and development.
From Early Intervention Support.com:
Failure to thrive is a weight consistently below the 3rd to the 5th percentile for age, progressive decrease in weight to below the 3rd to the 5th percentile, or a decrease in the percentile rank of 2 major growth parameters in a short period.
Conclusion: Help Guide says that children with weight in the 75th percentile are too heavy and if in the 95th percentile they are overweight. Early Intervention Support says that a baby is in the category of “failure to thrive” if it is in the 5th to 3rd percentile. Some mothers reported in various blogs that their babies had consistently lost weight until the weight was so low that they “fell of the charts” Therefore one must assume that the lower the percentile, the lower the weight and the higher the percentile, the higher the child’s weight.
The Term Percentile As Used by Stock Brokers
Here the terms “Top 10th Percentile” and “In the top 10%” are used to describe the same thing. According to the definition of percentiles researched there is no such thing as the “Top X percentile.” However, something or someone can be among the top 10 or score within the 10th percentile. The first expression therefore seems to be used incorrectly. See article headline below:
From FORBES Online:
"Top 10th Percentile Ranked Dividend Stock BGCP Becomes Oversold"
The Dividend Rank formula at Dividend Channel ranks a coverage universe of thousands of dividend stocks, according to a proprietary formula designed to identify those stocks that combine two important characteristics — strong fundamentals and a valuation that looks inexpensive. BGC Partners Class A (NASD: BGCP) presently has a stellar rank, in the top 10% of the coverage universe, which suggests it is among the top most "interesting" ideas that merit further research by investors.
Conclusion: Here, there are expressions that are technical terms or jargon, such as "oversold" and would require preparation before interpretation. I do not often hear ''top most interesting", rather top ten, or, most interesting.
The Term Percentile As Used by Teachers
A percentile indicates the relative standing of a data value when data are sorted into numerical order, from smallest to largest. p% of data values are less than or equal to the pth percentile.
Low percentiles always correspond to lower data values. High percentiles always correspond to higher data values.
A percentile may or may not correspond to a value judgment about whether it is "good" or "bad". The interpretation of whether a certain percentile is good or bad depends on the context of the situation to which the data applies. In some situations, a low percentile would be considered "good'; in other contexts a high percentile might be considered "good". In many situations, there is no value judgment that applies.
Understanding how to properly interpret percentiles is important not only when describing data, but is also important in calculating probabilities.
From How to Calculate Percentiles by an eHow Contributor:
(I tried this, as did several of my colleagues but obtained no successful results.)
When you take a test and get a score back 87%. It tells you how many questions you got right. But your test score doesn’t tell you how well you did compared to other people who took the test. Percentiles are values from 0 to 99 that tell you the percentage of tests with scores less than a particular score. If the percentile of your test score is 75, this means you scored higher than 75% of the people who took the test. Percentiles can be used to compare values in any set of data that is ordered. You can compute percentiles for income, weight, and many other things.
Things You'll Need:
Data set of observations (in this guide, we will assume a set of 150 test scores)
Computer spreadsheet or pencil and paper for sorting the data set
1. Sort the test scores so they are in order from lowest to highest score. Normally this is done by entering the scores in a computer spread sheet and then clicking on the sort command. You can do this manually by listing the possible scores on the test in order and then making a hash mark beside the appropriate score for each test.
2. Start to calculate the percentile of your test score (as an example we’ll stick with your score of 87). The formula to use is L/N(100) = P where L is the number of tests with scores less than 87, N is the total number of test scores (here 150) and P is the percentile. Count up the total number of test scores that are less than 87. We’ll assume the number is 113. This gives us L = 113 and N = 150.
3. Divide out L/N to get the decimal equivalent. (113/150 = 0.753). Multiply this by 100 (0.753(100) = 75.3).
4. Discard the digits to the right of the decimal point. For 75.3 this leaves 75. This is the percentile of a score of 87 and means you did better than 75% of the people who took the test. Not bad at all!
5. Calculate the score which is at a given percentile. Let’s say you want to know what the median test score is (the test score for which 50% of the students scored less and 50% scored as much or higher. We use the same variables but a slightly different equation. The formula is P/100(N) = L. In our example, P = 50 and N = 150 so we have 50/100(150) = 75.
6. Count the number of test scores starting with the lowest until you get to 75. The next higher score (#76) is the score at the 50th percentile.
Conclusion: After implementing the formula and reading thoroughly trying to understand what teachers mean when they say that a child is in the 90th percentile, I must say that I do not entirely understand it. It translates into Spanish as "percentil". When interpreters research vocabulary it helps to know when to say enough and when to keep digging.
7/79/2012: After reading the piece above several times, I remembered something.
Percentile. I don't remember teachers using the term percentile when I was a in elementary school, although that was a long time ago and I may have just forgotten. After reading this article, though, I did remember that my elementary school report card had printed in large, bold letters on the back: Do not compare your child’s grades with the grades of his or her classmates. The term percentile, as it is explained in the teachers'-use section above, sound like it means how well one did on a test compared to other people who took the same test. I am not sure I am very interested in understanding this method of ranking students in comparison to others. I make my children study for exams and expect them to obtain good grades. Although I want to know how students of one school compare to students of other schools (especially if an institution is private and/or costly) I, as a parent prefer to focus on how my children's grades reflect their efforts, performance during exams, success with certain methods of study, etc. If schools rank a student based on the average grades of its entire student body, in one school one child may be underestimated as an average student while he or she is in reality outstanding or, in a different school, a child may be average when in reality he or she may be failing. Most parents already observe their children's grades and try to understand their performance in school and help them, but I don't think I’m interested at this time in trying so hard to understand the method teachers use to calculate how a first grader ranked out of a group of maybe 1000 testers. I understand that the term percentile may be used to allocate Federal funding and to determine school-rankings within a State. In the future, when comparing colleges, the term percentile will probably come in handy in understanding average grades of the college population, percentage of student who reach graduation, undergraduates that obtained work, etc. For now, I think it is sufficient that teachers and administrators understand the term ‘percentile’ since it's their schools that are being graded. I would be satisfied if they just spoke to me in plain English.