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Question
Find the following table gives the distribution of the life time of 400 neon lamps:
| Life time (in hours) | Number of lamps |
| 1500 – 2000 | 14 |
| 2000 – 2500 | 56 |
| 2500 – 3000 | 60 |
| 3000 – 3500 | 86 |
| 3500 – 4000 | 74 |
| 4000 – 4500 | 62 |
| 4500 – 5000 | 48 |
Find the median life time of a lamp.
Solution
The cumulative frequencies with their respective class intervals are as follows.
| Life time | Number of lamps (fi) |
Cumulative frequency |
| 1500 − 2000 | 14 | 14 |
| 2000 − 2500 | 56 | 14 + 56 = 70 |
| 3000 − 3500 | 60 | 70 + 60 = 130 |
| 3000 − 3500 | 86 | 130 + 86 = 216 |
| 3500 − 4000 | 74 | 216 + 74 = 290 |
| 4000 − 4500 | 62 | 290 + 62 = 352 |
| 4500 − 5000 | 48 | 352 + 48 = 400 |
| Total (n) | 400 |
It can be observed that the cumulative frequency just greater than `n/2 (i.e 400/2 = 200)` is 216
belonging to class interval 3000 − 3500.
Median class = 3000 − 3500
Lower limit (l) of median class = 3000
Frequency (f) of median class = 86
Cumulative frequency (cf) of class preceding median class = 130
Class size (h) = 500
Median = `l +((n/2 -cf)/f)xxh`
= `3000+ ((200-130)/86)xx500`
= `3000+(70xx500)/86`
= 3406.976
Therefore, the median life time of lamps is 3406.98 hours.
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