Python – Sort data before using numpy.median


I'm measuring the median and percentiles of a sample of data using Python.

import numpy as np
x25=np.percentile(data, 25)
x75=np.percentile(data, 75)

Do I have to use the np.sort() function on my data before measuring the median?

Best Solution

According to the documentation of numpy.median, you don't have to manually sort the data before feeding it to the function, as it does this internally. It is actually very good practice to view the source-code of the function, and try to understand how it works.

Example, showing that sorting beforehand is unnecessary:

In [1]: import numpy as np

In [2]: data = np.array([[ 10, 23,  1,  4,  5],
   ...:                  [  2, 12,  5, 22, 14]])

In [3]: median = np.median(data)  # Median of unsorted data

In [4]: median
Out[4]: 7.5

In [5]: data.sort()  # Sorting data

In [6]: median_sorted = np.median(data.ravel())  # Median of the flattened array

In [7]: median_sorted
Out[7]: 7.5

In [8]: median == median_sorted  # Check that they are equal
Out[8]: True