# 12 Bootstrap Confidence Intervals

**1. MEANS**

**Bootstrap Confidence Interval for:**

Confidence Interval for a Meannew window | Difference in Meansnew window

**STEP 1:** Enter the original sample data into StatKey by clicking on **Edit Data**.

Manually edit the values or paste a tab or comma seperated file into the box and click Ok.

If updating a data file, ensure there’s only one column (or two if there is an identifier).

Note: Data entry and confidence interval calculation process for a difference in means is similar.

**STEP 2:** Generate several thousand samples (say, 10,000 samples) by clicking on the **Generate 1000 Samples** button several times.

If you’re calculating Bootstrap confidence intervals by plugging in the standard error, use the value of

std. errorshown on the Bootstrap dotplot.

ForPERCENTILEMethod, continue with the following steps.

**STEP 3: Select TWO-TAIL for confidence intervals. **

**STEP 4:** Change the middle area/proportion to match your confidence level.

*After STEP 4, the lower and upper bounds of your confidence interval are shown below the horizontal axis on the main graph. See image below.*

*Calculator Screenshot*

**2. PROPORTIONS**

**Bootstrap Confidence Interval for:**

Proportionnew

window | Difference in Proportionsnew

window

**Note**: Data entry and confidence interval calculation process for a**difference in proportions**is similar.**STEP 1:** Enter the original sample data into StatKey by clicking on **Edit Data**. Enter the sample size and the count/frequency for each sample in the dialog box.

**STEP 2:** Generate several thousand samples (say, 10,000 samples) by clicking on the **Generate 1000 Samples** button several times.

If you’re calculating Bootstrap confidence intervals by plugging in the standard error, use the value of std. error shown on the Bootstrap dotplot.

ForPERCENTILE Method, continue with the following steps.

**STEP 3:** Select **TWO-TAIL** for confidence intervals.

**STEP 4:** Change the middle area/proportion to match your confidence level.

*After STEP 4, the lower and upper bounds of your confidence interval are shown below the horizontal axis on the main graph. See image below.*

*Calculator Screenshot*