Which interval is wider

Significance tests on their own do not provide much light about the nature or magnitude of any effect to which they apply. One way of shedding more light on those issues is to use confidence intervals. Confidence intervals can be used in univariate, bivariate and multivariate analyses and meta-analytic studies. A narrow confidence interval enables more precise population estimates.

The width of the confidence interval is a function of two elements:. If we assume the confidence level is fixed, the only way to obtain more precise population estimates is to minimize sampling error. Sampling error is measured by the standard error statistic. The size of the standard error is due to two elements:. One thing we can do is to increase the sample size. As a general guide, to halve the standard error the sample size must be quadrupled.

However, in cases where such precision is not required there is a point where the gain in precision is not worth the cost of increasing the size of the sample. The figures in Table 1 below were obtained for the average income of males and females in a fictitious survey for unemployment. A confidence interval is a range around a measurement that conveys how precise the measurement is.

For most chronic disease and injury programs, the measurement in question is a proportion or a rate the percent of New Yorkers who exercise regularly or the lung cancer incidence rate. Confidence intervals are often seen on the news when the results of polls are released. This is an example from the Associate Press in October Emphasis added.

Although it is not stated, the margin of error presented here was probably the 95 percent confidence interval. In the simplest terms, this means that there is a 95 percent chance that between Conversely, there is a 5 percent chance that fewer than For any given sample size, dispersion and confidence level, a one-tailed confidence "interval" is "narrower" than a two tailed interval in the sense that the distance from the observed effect to the computed boundary is smaller for the one-tailed interval the one-tailed case is not really an interval, since it has only one boundary.

As was the case with power analysis, however, the decision to work with a one-tailed procedure rather than a two-tailed procedure should be made on substantive grounds, rather than as a means for yielding a more precise estimate of the effect size.

Previous Next. The point estimate 0. The confidence interval describes the uncertainty inherent in this estimate, and describes a range of values within which we can be reasonably sure that the true effect actually lies. If the confidence interval is relatively narrow e.

If the interval is wider e. Intervals that are very wide e. This statement is a loose interpretation, but is useful as a rough guide.