# Confidence interval for event rate

I have a data set where I am counting how many events happen ($N$) over a given duration ($t$), and am defining an event rate as $R=N/t$. What I'm wondering is what $N$ gives me a certain confidence on the rate. Looking on the web I found this formula:

$$\Delta R = Z\sqrt{\frac{R}{t}},$$ or $$N = (\frac{t\Delta R}{Z})^2$$

But, if this is the $N$ required to get a certain confidence interval on my rate, this equation means the smaller I want my confidence interval, the smaller $N$ becomes. This seems counter-intuitive, I feel like I have more confidence when $N$ is large.

Eg. if I am counting people walking by, and I count 3 in 60 minutes, that is 1/20 per minute and I don't have much confidence in that number. If, however, I saw 300 people in 60 minutes, then I am more confident that they come by at 5 per minute.

• Your intuition is concerned with the relative magnitude of the interval width, which is proportional to the ratio of $\sqrt N$ to $N:$ indeed, that does get smaller as $N$ increases. – whuber Jun 21 '18 at 1:18
• You didn't explain your symbol $Z$. – kjetil b halvorsen May 5 '19 at 12:33

(You didn't explain $$Z$$, please do so.) If we assume a Poisson distribution for your count $$N_i$$ (is is unclear if you have one or more counts) then $$N_i \sim \mathcal{Po}(\lambda t_i)$$ and the observed rate $$R_i=N_i/t_i$$ will have expectation $$\lambda$$, the parameter representing the rate. The standard deviation of $$R_i$$ will be $$\sqrt{\lambda/t_i}$$, and that could be used for an approximate confidence interval. $$N$$ is the count, an observation, you cannot choose it. If the rate $$\lambda$$ is large, then since expectation and variance in the Poisson distribution are equal, the standard deviation of the rate estimate becomes larger. What you can choose is the observation interval $$t_i$$, and by making that larger you can reduce the standard deviation.

It could be better to use the loglikelihood function and construct a CI directly from it, inverting the likelihood ratio test (This is effectively what is done in the R example below.)

If you have $$n$$ independent observations, say counts on different days, you could use Poisson regression, with the (log of) the observation time $$t_i$$ as an offset. In R code this could look like, using simulated data:

set.seed(7*11*13)
t <- c(1, 2, 3, 3, 3, 2, 5, 2, 3, 8)
lambda <- 3
N <- rpois(length(t), lambda*t)
yourdf <- data.frame(t, N)
mod <- glm(N ~ offset(log(t)), family=poisson, data=yourdf)
mod

Call:  glm(formula = N ~ offset(log(t)), family = poisson, data = yourdf)

Coefficients:
(Intercept)
1.159

Note that $$e^{1.159} \approx 3.19$$ close to the value used in simulation $$\lambda=3$$. The we can get a confidence interval on $$\lambda$$ by

exp(confint(mod))
Waiting for profiling to be done...
2.5 %   97.5 %
2.608252 3.846705