Bug 201582

Summary:

ministat does not calculate proper median value (patch included)

Product:

Base System

Reporter:

Marcus Reid <marcus>

Component:

bin

Assignee:

Marcelo Araujo <araujo>

Status:

Closed FIXED

Severity:

Affects Many People

CC:

araujo, emaste

Priority:

---

Version:

CURRENT

Hardware:

Any

OS:

Any

Attachments:

Description	Flags
Make ministat return proper value for median	none

Description Marcus Reid 2015-07-15 07:24:48 UTC

Created attachment 158793 [details]
Make ministat return proper value for median

From http://www.mathgoodies.com/lessons/vol8/median.html :

"The median of a set of data is the middlemost number in the set. The median is also the number that is halfway into the set. To find the median, the data should be arranged in order from least to greatest. If there is an even number of items in the data set, then the median is found by taking the mean (average) of the two middlemost numbers."

Ministat currently returns the second of the two middle numbers if there is an even number of entries.  This patch takes the middle two and returns their average.

*** usr.bin/ministat/ministat.c.orig	2015-07-14 23:49:11.246171000 -0700
--- usr.bin/ministat/ministat.c	2015-07-15 00:16:20.895494000 -0700
***************
*** 193,199 ****
  Median(struct dataset *ds)
  {
  
! 	return (ds->points[ds->n / 2]);
  }
  
  static double
--- 193,200 ----
  Median(struct dataset *ds)
  {
  
! 	if(!(ds->n % 2)) return ((ds->points[ds->n / 2]) + (ds->points[(ds->n / 2)-1]))/2;
! 	else return (ds->points[ds->n / 2]);
  }
  
  static double

Comment 1 Marcelo Araujo freebsd_committer

2015-11-21 03:54:38 UTC

I will take it.

Comment 2 Marcelo Araujo freebsd_committer

2015-11-23 04:30:32 UTC

Hi,

ministat(1) actually is doing in the right way.
I made couple tests with other computer languages and they return the same value as ministat returns.

As a simple example using two different ways in Python:
>>> import statistics
>>> items = [1,2,13,4,5,6,7]
>>> statistics.median(items)
4

Also I made my own:
>>> def middle(L):
...     L = sorted(L)
...     n = len(L)
...     m = n -1
...     return (L[n/2] + L[m/2]) / 2.0
... 
>>> print middle(items)
4.0

Ministat result:
    N           Min           Max        Median           Avg        Stddev
x   7             1             7             4             4     2.1602469
+   7             1             7             4             4     2.1602469

So, I can't see where is the problem.


Best,

Comment 3 Marcus Reid 2015-11-23 06:27:59 UTC

You're not testing for the problem case there.  Here, let me demonstrate:

>>> import statistics
>>> items = [1,2,3,4]
>>> statistics.median(items)
2.5


[mreid@sol /usr/home/mreid]$ ministat
1
2
3
4
x <stdin>
+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
|x                                                                          x                                                                           x                                                                          x|
|                |________________________________________________________________________________________________A_____________________________________M__________________________________________________________|                |
+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
    N           Min           Max        Median           Avg        Stddev
x   4             1             4             3           2.5     1.2909944


As you can see, ministat has a median of 3 whereas the real median is 2.5, as seen in the python example.

It's the case where there is no middle number (an even number of items in the dataset) that is broken.

Thanks!

Comment 4 Marcelo Araujo freebsd_committer

2015-11-23 06:36:57 UTC

Thanks for the feedback.
Yes, you are right with this case!

Comment 5 commit-hook freebsd_committer

2015-11-24 02:32:02 UTC

A commit references this bug:

Author: araujo
Date: Tue Nov 24 02:30:59 UTC 2015
New revision: 291231
URL: https://svnweb.freebsd.org/changeset/base/291231

Log:
  Compute the median of the data set as the midpoint between the two middle
  values when the data set has an even number of elements.

  PR:		201582
  Submitted by:	Marcus Reid <marcus@blazingdot.com>
  Reviewed by:	imp
  Approved by:	bapt (mentor)

Changes:
  head/usr.bin/ministat/ministat.c

Comment 6 Marcelo Araujo freebsd_committer

2015-11-24 02:32:47 UTC

Committed thanks!