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Wilcoxon Signed Rank Test
• Last Updated : 26 Nov, 2020

Prerequisites: Parametric and Non-Parametric Methods
Hypothesis Testing

Wilcoxon signed-rank test, also known as Wilcoxon matched pair test is a non-parametric hypothesis test that compares the median of two paired groups and tells if they are identically distributed or not.

We can use this when:

Differences between the pairs of data are non-normally distributed.
Independent pairs of data are identical. (or matched) Eg. (Math, English: Both subjects) ; (June, July: Both months)

Steps involved:

```Step 1 - Determine the null (h0) and alternate (ha) hypothesis.
Step 2 - Find the difference (D) between the two columns. [D = B-A]
Step 3 - Find absolute difference (Abs-D). [Abs-D = |D|]
Step 4 - Assign ranks to Abs-D from lowest (1) to highest (n).```

Assigning Ranks:

If any two or more Abs-D values are same, then assign them consecutive ranks, then find the average of the ranks for each set of duplicate value. Consider the following scenario:

```Case I - For Abs-D = 3
-> Assign them consecutive possible ranks. (3,4)
-> Find average of 3,4 => (3+4)/2 = 3.5
-> Assign the rank = 3.5 to both the 3's present in the table.             ```
```Case II - For Abs-D = 4
-> Assign them consecutive possible ranks. (5,6,7)
-> Find average of 5,6,7 => (5+6+7)/3 = 6
-> Assign the rank = 6 to all the 4's present in the table.```
```Step 5 - Find the sum of the ranks assigned to positve (T+) and negative (T-) Abs-D values.
Step 6 - Find the Wilcoxon Rank. (Wcalc = minimum(T+,T-))
Step 7 - Use the value of n and α and find Wtable in two-tailed section of
'Critical values of wilcoxon signed rank test'.
(take α = 0.05, if not given)
Step 8 - Interpretation of result.```

NOTE : We use two-tailed test when we are dealing with two hypothesis. (null and alternate)

Interpretation of result

```When Wcalc < Wtable :
-> Reject H0 (null hypothesis)
-> The two groups are not identically distributed.

When Wcalc > Wtable :
-> Accept H0 (null hypothesis)
-> The two groups are identically distributed.```

Example Problem (Step by Step):

Consider the following example. The smog concentration data of 13 states of India were measured. Perform the Wilcoxon signed rank test and determine if there’s a significant difference in the concentrations recorded in May to that in December. [take α = 0.05]

```n = 13
α = 0.05

Step 1 - h0 : Cmay = Cdecember (no change in the smog concentration)
h1 : Cmay ≠ Cdecember (smog concentration changed)```
`Step 2,3,4 - Refer the table given above.`
```Step 5 - T+ marked as [ ] in table.
T- marked as [ ] in table.
∑T+ = 75
∑T- = 16```
```Step 6 - Wcalc = minimum(75,16)
= 16```
```Step 7 - Using n = 13 and α = 0.05 in table (click here)
Wtable = 17```
```Step 8 - Wcalc < Wtable :
Rejecting H0.
i.e smog concentration have changed from before.```

Conclusion:

Wilcoxon signed-rank test is a very common test in the fields of pharmaceuticals, especially amongst drug researchers, to find out the dominant symptoms of various drugs on humans. Being a non-parametric test, it works as an alternative to T-test which is parametric in nature. For any doubt/query, comment below. My Personal Notes arrow_drop_up