Question

We are given a jar full of thousands of red and blue marbles. We want to estimate the unknown proportion pof red marbles in the jar. To do this, we randomly draw 100 marbles and count reds: it so happens we drew 45 reds. Enter values in decimal form, rounded to four decimal places (or more).

We estimate the proportion of reds in the jar to be
Attach a give-or-take value to this estimate. (That is, estimate the standard error.)
For a 96% confidence interval, about how many standard errors should be added to and subtracted from the estimate?
Set up an approximate 96% confidence interval for the unknown proportion of reds in the jar.

Answer 1

(0.3478, 0.5522)

Explanation:

Given:

Total number of red marbles, x = 45

Total number of marbles, n = 100

Phat = x / n = 45 / 100 = 0.45

The confidence interval, C.I is given by :

Phat ± Zcritical * standard error

Phat ± Zcritical * √Phat(1 - Phat) / n

Zcritical at 96% = 2.0537

The standard error = √Phat(1 - Phat) / n

S.E = √(0.45 * 0.55) / 100 = 0.0497493

C.I = 0.45 ± (2.0537 * 0.0497493)

C.I = 0.45 ± 0.10217013741

C. I = (0.3478, 0.5522)