# Specify the confidence and prediction intervals for Predict

To perform this analysis in Minitab, go to the menu that you used to fit the model, then choose Predict > Options. For more information, go to Stored model overview.

## Confidence level

From Confidence level, select the level of confidence for the confidence intervals and the prediction intervals.

Usually, a confidence level of 95% works well. A 95% confidence level indicates that, if you took 100 random samples from the population, the confidence intervals for approximately 95 of the samples would contain the mean response. Similarly, the prediction interval indicates that you can be 95% confident that the interval contains the value of a single new observation.

For a given set of data, a lower confidence level produces a narrower interval, and a higher confidence level produces a wider interval. The width of the interval also tends to decrease with larger sample sizes. Therefore, you may want to use a confidence level other than 95%, depending on your sample size.
• If your sample size is small, a 95% confidence interval may be too wide to be useful. Using a lower confidence level, such as 90%, will produce a narrower interval. However, the likelihood that the interval contains the mean response decreases.
• If your sample size is large, you may want to consider using a higher confidence level, such as 99%. With a large sample, a 99% confidence level may produce a reasonably narrow interval and also increase the likelihood that the interval contains the mean response.

## Type of interval

From Type of interval, select a two-sided interval or a one-sided bound. For the same confidence level, a bound is closer to the point estimate than the interval. The upper bound does not give a likely lower value. The lower bound does not give a likely upper value.

For example, the predicted mean concentration of dissolved solids in water is 13.2 mg/L. The 95% confidence interval for the mean of multiple future observations is 12.8 mg/L to 13.6 mg/L. The 95% upper bound for the mean of multiple future observations is 13.5 mg/L, which is more precise because the bound is closer to the predicted mean.

Two-sided
• Use a two-sided confidence interval to estimate both likely upper and lower values for the mean response.
• Use a two-sided prediction interval to estimate both likely upper and lower values for a single future observation.
Lower bound
• Use a lower confidence bound to estimate a likely lower value for the mean response.
• Use a lower prediction bound to estimate a likely lower value for a single future observation.
Upper bound
• Use an upper confidence bound to estimate a likely higher value for the mean response.
• Use an upper prediction bound to estimate a likely higher value for a single future observation.
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