Difference based series are very common in the IBM IPAT. The good news is that with practice they are very easy to master. As the name suggests, the difference between consecutive numbers will yield the clue to decoding the series.

Difference based series tested in the IPAT can largely be classified into four types-

**1] Constant difference series**

3 7 11 15 19 23 ?

In this form, the difference between consecutive numbers is a constant. An examination of the difference yields a constant of 4 (23-19 = 19-15 = 4). Hence the next number in the series is 23 + 4 = 27.

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*2] Increasing or decreasing difference series

94 194 295 397 ?

In this form, the difference between consecutive numbers increases or decreases by a fixed amount

As we observe,

194 – 94 = 100

295 – 194 = 101

397 – 295 = 102

Hence, the next number in the series is 397 + 103 = 500.

**3] Multi layer difference series**

37 41 49 61 77 ?

In this form, the difference between consecutive numbers increases or decreases in a relative proportion. Here we see,

41 – 37 = 4

49 – 41 = 8

61 – 49 = 12

77 – 61 = 16

A handy trick is to take the **difference of differences**: 8 – 4 = 4, 12 – 8 = 4, 16 – 12 = 4. This multi-layer constancy or dependency characteristic gives the name to the series.

**4] Compound series**

12 14 20 32 52 82 ?

Many difference based series in the IPAT cannot be classified in the above three types. The difference between the series rises and falls non linearly.

Here the consecutive differences are 2,6,12,20,30. These are part of the series 1×2,2×3,3×4,4×5,5×6. Tricky, isnt it? No worries, with adequate practice, you will be able to identify and relate back such patterns with considerable ease. We have developed a comprehensive IPAT numeric series practice book** **which you can find here.

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