x Turn on thread page Beta
 You are Here: Home >< Maths

# S2 Poisson struggles - lightning strikes watch

1. In a country the mean number of deaths per year from lightning strike is 2.2.

First part of question ask to find probability of 0,1,2 >2 deaths. That was easy. X ~ Poisson(2.2) and calculate probabilities. I had no problem with that.

But next part of question is as follows.

In a neighbouring country, it is found that one year in 20 nobody dies from lightning strike. What is the mean number of deaths per year in that country from lightning strike?

Thinking through this one, do they mean that because it is neighbouring that mean per year is still 2.2 but every 20th year there is no death. so 19 x 2.2 + 0 = 41.8. 41.8/20 = 2.09 which is mean for this neighbouring country. But that in fact is incorrect answer. answer should be 3.

I know from question that in neighbouring country 1 year in 20 there are no deaths. But I don't know how many deaths there are in the other 19 years in 20. So how is it possible to work this out? What am I missing here?
2. (Original post by acomber)

In a neighbouring country, it is found that one year in 20 nobody dies from lightning strike. What is the mean number of deaths per year in that country from lightning strike?

Thinking through this one, do they mean that because it is neighbouring that mean per year is still 2.2 but every 20th year there is no death. so 19 x 2.2 + 0 = 41.8. 41.8/20 = 2.09 which is mean for this neighbouring country. But that in fact is incorrect answer. answer should be 3.

I know from question that in neighbouring country 1 year in 20 there are no deaths. But I don't know how many deaths there are in the other 19 years in 20. So how is it possible to work this out? What am I missing here?
Assuming it's Poisson again, then you have P(X=0)=0.05.

So, what value of lambda gives that. I.e. what's the formula for P(x=0) in terms of lambda. And solve.
3. (Original post by ghostwalker)
Assuming it's Poisson again, then you have P(X=0)=0.05.

So, what value of lambda gives that. I.e. what's the formula for P(x=0) in terms of lambda. And solve.
Thank you.

ie

let l = lambda

log(e^-l) = log(0.05)

-l log(e) = log(0.05)

lambda = -log(0.05)/loge = 3

Turn on thread page Beta
TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: January 8, 2015
Today on TSR

### Negatives of studying at Oxbridge

What are the downsides?

Poll
Useful resources

## Make your revision easier

### Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

### How to use LaTex

Writing equations the easy way

### Study habits of A* students

Top tips from students who have already aced their exams

Can you help? Study help unanswered threads

## Groups associated with this forum:

View associated groups

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE