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Cogito

Probability and Statistics · Chapter 8 · Lesson 1

Practice Worksheet

Confidence Intervals

A range, and how sure you are of it.

12 problems · about 22 minutes · TEKS P.S.4.D, S-IC.B.4

NameDate

Figure — use these to answer the problems

Slide the estimate and picture a band either side of it. That band is the confidence interval.
05101520253035404550556065707580859095100

55

Estimate, in percent55
Figure 1

Warm Up

Straightforward practice. Get the method working first.

5 problems

  1. Estimate 55% with margin 3. What is the upper end, in percent?

    Answer

  2. How does a larger sample change the interval?

    • It narrows it, since the margin of error shrinks.
    • It widens it.

  3. Estimate 45% with margin 4. Upper end, in percent?

    Answer

  4. Estimate 30% with margin 5. Lower end?

    Answer

  5. Interval 38% to 46%. What is the estimate?

    Answer

Build It Up

The same ideas with more to keep track of.

3 problems

  1. Interval 38% to 46%. Margin of error?

    Answer

  2. 450 of 900 said yes. Sample percentage?

    Answer

  3. Quadrupling the sample size does what to the margin? 1 halves it, 2 quarters it.

    Answer

Stretch Yourself

Mixed problems. Work out what kind of question it is before you start.

4 problems

  1. Which interpretation of 95% confidence is correct?

    • About 95% of intervals built this way capture the true value.
    • There is a 95% chance the true value is in this particular interval.

  2. Estimate 72% with margin 6. Lower end?

    Answer

  3. The Interval

    Estimate 60% with margin 3. What is the lower end, in percent?

    Answer%

  4. The Width

    An interval runs from 42% to 50%. What is the margin of error, in percentage points?

    Answer