# What is the effect of increasing the level of confidence on the width of the interval?

Increasing the sample size decreases the width of confidence intervals, because it decreases the standard error. c) The statement, "the 95% confidence interval for the population mean is (350, 400)", is equivalent to the statement, "there is a 95% probability that the population mean is between 350 and 400".
A.

### Is a 95% confidence interval wider than a 90% confidence interval?

A 99 percent confidence interval would be wider than a 95 percent confidence interval (for example, plus or minus 4.5 percent instead of 3.5 percent). A 90 percent confidence interval would be narrower (plus or minus 2.5 percent, for example).
• #### Why are some confidence intervals wider?

Apparently a narrow confidence interval implies that there is a smaller chance of obtaining an observation within that interval, therefore, our accuracy is higher. Also a 95% confidence interval is narrower than a 99% confidence interval which is wider. The 99% confidence interval is more accurate than the 95%.
• #### What does it mean when the confidence interval is negative?

The 95% confidence interval is providing a range that you are 95% confident the true difference in means falls in. Thus, the CI can include negative numbers, because the difference in means may be negative.
• #### What do standard error bars tell you?

Error bars are graphical representations of the variability of data and used on graphs to indicate the error or uncertainty in a reported measurement. They give a general idea of how precise a measurement is, or conversely, how far from the reported value the true (error free) value might be.
B.

### What is a good 95% confidence interval?

If repeated samples were taken and the 95% confidence interval was computed for each sample, 95% of the intervals would contain the population mean. A 95% confidence interval has a 0.95 probability of containing the population mean. 95% of the population distribution is contained in the confidence interval.
• #### Which confidence interval is wider?

A 99 percent confidence interval would be wider than a 95 percent confidence interval (for example, plus or minus 4.5 percent instead of 3.5 percent). A 90 percent confidence interval would be narrower (plus or minus 2.5 percent, for example).
• #### What is the significance of the width of the confidence interval?

Increasing the sample size decreases the width of confidence intervals, because it decreases the standard error. c) The statement, "the 95% confidence interval for the population mean is (350, 400)", is equivalent to the statement, "there is a 95% probability that the population mean is between 350 and 400".
• #### What happens to the margin of error if you increase the confidence level?

Answer: As sample size increases, the margin of error decreases. As the variability in the population increases, the margin of error increases. As the confidence level increases, the margin of error increases.
C.

### How can the width of a confidence interval be reduced?

1. Increase the sample size. Often, the most practical way to decrease the margin of error is to increase the sample size.
2. Reduce variability. The less that your data varies, the more precisely you can estimate a population parameter.
3. Use a one-sided confidence interval.
4. Lower the confidence level.
• #### What happens to the width of the confidence interval as the confidence level increases?

The width of the confidence interval decreases as the sample size increases. The width increases as the standard deviation increases. The width increases as the confidence level increases (0.5 towards 0.99999 - stronger). The width increases as the significance level decreases (0.5 towards 0.0000001 - stronger).
• #### Is a smaller standard error better?

If you measure a sample from a wider population, then the average (or mean) of the sample will be an approximation of the population mean. The smaller the standard error, the less the spread and the more likely it is that any sample mean is close to the population mean. A small standard error is thus a Good Thing.
• #### How can the width of a confidence interval be reduced?

1. Increase the sample size. Often, the most practical way to decrease the margin of error is to increase the sample size.
2. Reduce variability. The less that your data varies, the more precisely you can estimate a population parameter.
3. Use a one-sided confidence interval.
4. Lower the confidence level.

Updated: 3rd December 2019