Prediction interval for sampled count data I am trying to get prediction intervals around a sampled count variable. 
For example, say I want to know the number of letters an apartment building receives every day. Each day I record the count from ten different apartments in the building, out of 100 total apartments, and get for example (toy data) [0, 5, 2, 0, 0, 1, 4, ...], which would represent seven days worth of total letters for those ten apartments. I can collect this over many days. (Note: I realize mail isn't delivered on Sundays, etc, but for now let's just assume it's all the same.) Let's assume this data comes from a Poisson distribution. I know I can get lambda from the data (just take mean). What I'm not sure of is how to calculate those prediction intervals for the full building. In my example, I have sampled only 10% of the building. Do I just multiply the limits of my intervals by 10? I suspect no, but I'm not sure where to make the change. 
I want to be able to say, for example, "on this day, 9 letters were delivered to my sample apartments, so I think x many letters were delivered to the whole building, and my 95% prediction interval is (x-y, x+y)". Where my guess is x = 90, but I'm not sure how to get y. 
Note: this originally asked about confidence intervals but I was advised to change it being that that isn't really what I was asking. 
 A: If each sampled apartment receives letters at a daily rate $\lambda$, then for $n$ apartments over $t$ days
$$\sum_{j=1}^{n_X} \sum_{i=1}^{t_X} X_{ij} = S_X \sim\mathsf{Pois}\left({n_X t_X\lambda}\right)$$
where $X_{ij}$ is the no. letters received on the $i$th day by the $j$th apartment sampled. Let $Y_{ij}$ be the no. letters received on the $i$th day by the jth unsampled apartment, & suppose for the moment that the daily rate might differ by a factor $\theta$, so that:
$$\sum_{j=1}^{n_Y} \sum_{i=1}^{t_X} Y_{ij} = S_Y \sim\mathsf{Pois}\left(n_Y t_Y\theta\lambda\right)$$
Considering the joint distribution of $S_X$ & $S_Y$, if you can find an $\alpha$-level test for $H_0: \theta=1$ vs $H_1: \theta\neq1$, you can invert it to give a set of $(S_X, S_Y)$ pairs for which the null is not rejected; & given $S_X=s_X$, a $1-\alpha$ prediction interval for $S_Y + s_X$. The test needs to not depend on the unknown common rate under the null, $\lambda$, which can be achieved by conditioning on the sufficient statistic for $\lambda$, $S_X+S_Y$:
$$S_Y|S_X=s_X \sim \mathsf{Binom}\left(S_X + S_Y, \frac{n_Y  t_Y}{n_X t_X + n_Y t_Y}\right)$$
Here is an illustration in R (I've used the equal-tailed test but you can substitute any exact two-sided test):
no.letters.sampled <- 12
no.apts.sampled <- 10
period.sampled <- 7
no.apts.unsampled <- 90
period.unsampled <- 7

no.letters.unsampled <- 0:200 # may need to widen search

calc.p.value <- Vectorize(function(s_X, n_X, t_X, s_Y, n_Y, t_Y){
  pi0 <- n_Y * t_Y / (n_X * t_X + n_Y *t_Y)
  min(1, 2*min(
    pbinom(s_Y-1, s_X + s_Y, pi0, lower.tail = FALSE),
    pbinom(s_Y, s_X + s_Y, pi0, lower.tail = TRUE)
  ))
})

p.value <- calc.p.value(
  no.letters.sampled, no.apts.sampled, period.sampled,
  no.letters.unsampled, no.apts.unsampled, period.unsampled
)

confidence.level <- 0.90
conf.region <- (no.letters.unsampled + no.letters.sampled)[p.value > 1 - confidence.level]
(conf.interval <- c(min(conf.region), max(conf.region)))
# returns a 95% C.I. of (72, 190)
plot(
  no.letters.unsampled + no.letters.sampled, 1-p.value, type="l",# lty=2,
  xlab="predicted no. letters", ylab="confidence level"
)
abline(h=confidence.level, col="dodgerblue")
text(120, 0.95, "90%", col="dodgerblue")


