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First, use the Critical Significance Level (α: alpha) glossary item: Critical Significance Level chosen in Step 2 and samples sizes (n1 and n2) calculated in Step 3 to find the Critical Value glossary item: Critical Value of U (Ucritical) using a Critical Value Table such as the one below e.g., if α = 0.05, n1 = 20 and n2 = 20 then Ucritical = 127.

Second, compare Ucritical with the value for the U statistic calculated in Step 3.

Reject your Null Hypothesis glossary item: Null Hypothesis if your calculated value is less than or equal to the critical value; U ≤ Ucritical (significant result).

Accept your Null Hypothesis glossary item: Null Hypothesis if your calculated value is greater than the critical value; U > Ucritical (non-significant result).

E.g., if U = 121 and Ucritical = 127 then reject the Null Hypothesis.

 

Table of Critical Values for a Critical Significance Levels (α: alpha) of 0.05 for the U statistic (smaller value) where n1 is the size of the first sample and n2 is the size of the second sample.
n1    1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20
n2 1                                        
  2               0 0 0 0 1 1 1 1 1 2 2 2 2
  3         0 1 1 2 2 3 3 4 4 5 5 6 6 7 7 8
  4       0 1 2 3 4 4 5 6 7 8 9 10 11 11 12 13 13
  5     0 1 2 3 5 6 7 8 9 11 12 13 14 15 17 18 19 20
  6     1 2 3 5 6 8 10 11 13 14 16 17 19 21 22 24 25 27
  7     1 3 5 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34
  8   0 2 4 6 8 10 13 15 17 19 22 24 26 29 31 34 36 38 41
  9   0 2 4 7 10 12 15 17 20 23 26 28 31 34 37 39 42 45 48
  10   0 3 5 8 11 14 17 20 23 26 29 33 36 39 42 45 48 52 55
  11   0 3 6 9 13 16 19 23 26 30 33 37 40 44 47 51 55 58 62
  12   1 4 7 11 14 18 22 26 29 33 37 41 45 49 53 57 61 65 69
  13   1 4 8 12 16 20 24 28 33 37 41 45 50 54 59 63 67 72 76
  14   1 5 9 13 17 22 26 31 36 40 45 50 55 59 64 67 74 78 83
  15   1 5 10 14 19 24 29 34 39 44 49 54 59 64 70 75 80 85 90
  16   1 6 11 15 21 26 31 37 42 47 53 59 64 70 75 81 86 92 98
  17   2 6 11 17 22 28 34 39 45 51 57 63 67 75 81 87 93 99 105
  18   2 7 12 18 24 30 36 42 48 55 61 67 74 80 86 93 99 106 112
  19   2 7 13 19 25 32 38 45 52 58 65 72 78 85 92 99 106 113 119
  20   2 8 13 20 27 34 41 48 55 62 69 76 83 90 98 105 112 119 127