Confidence Intervals
Calculation Steps
- choose the population statistic
- find its sampling distribution
- decide on level of confidence
- find the confidence limits
Cheat Sheet
| Population Statistic | Population Distribution | Conditions | Confidence Interval |
|---|---|---|---|
| Normal | You know what |
$$(\bar{x}-c\frac{\sigma }{\sqrt{n}},\bar{x}+c\frac{\sigma }{\sqrt{n}})$$ | |
| Non-normal | You know what |
$$(\bar{x}-c\frac{\sigma }{\sqrt{n}},\bar{x}+c\frac{\sigma }{\sqrt{n}})$$ | |
| Normal or Non-normal | You don’t know what |
$$(\bar{x}-c\frac{s}{\sqrt{n}},\bar{x}+c\frac{s}{\sqrt{n}})$$ | |
| Binomial | $$(p_{s}-c\sqrt{\frac{p_{s}q_{s}}{n}},p_{s}+c\sqrt{\frac{p_{s}q_{s}}{n}})$$ | ||
| Normal or Non-normal | You don’t know what |
$$(\bar{x}-t(v)\frac{s}{\sqrt{n}},\bar{x}+t(v)\frac{s}{\sqrt{n}})$$ | |
| where |
| Level of Confidence | Value of |
|---|---|
| 90% | 1.64 |
| 95% | 1.96 |
| 99% | 2.58 |
| and |
interval in general
statistic ± (margin of error)
margin of error = c * (standard deviation of statistic)
Sources: 1