GraphPad - FAQ 203 - Why is the 95% confidence interval of the X intercept of linear regression asymmerical?
Why is the 95% confidence interval of the X intercept of linear regression asymmerical?
This asymetry will be very noticeable if you only have a few points with lots of scatter, and will be almost unnoticeable with lots of points with little scatter.
GraphPad Prism 4 (but not Prism 3) reports the 95% confidence interval of the X intercept if you check an option on the Linear regression parameters dialog. Because the uncertainty is not symmetrical, it rarely makes sense to report a standard error of the X-intercept. Prism will report the best-fit value of the X intercept along with a SE and 95% confidence interval.
Objective:This section will explain the meaning of the Confidence Interval (CI) in statistical analysis.
The CI states that with 92% confidence, the proportion of all similar companies with the plan will between 46% and 56%. To copy selected text, right click to Copy or choose the Copy option under your browser's Edit menu. Text copied in this manner can be pasted directly into most documents with formatting maintained.

For a 95% confidence interval, with the population standard deviation unknown, but with the number of samples n>30, we use the student t distribution. Somebody please help me I need to create a confidence interval for a data set I have but I have absolutely no idea how or where to begin. Now, let's construct a 95% confidence interval to estimate the previous population proportion.
Why doesn't linear regression report a standard error of the X intercept, like it does for the Y intercept? The region beteen the 95% confidence bands for the best fit line (blue curves) is the 95% CI of the X intercept. It is much better to report both ends of the 95% confidence interval, which Prism 4 shows you.
Often, statistics are not expressed in terms of one number but rather as a range or an interval with a given level of confidence. I had actually began a report that after looking at your example was probably more complicated than it needed to be.
This interval will be computed from the SE value so will be symmetrical around the X intercept, and it won't be as accurate as the asymmetrical interval reported by linear regression. For proportions, the normal distribution approximates the binomial for n x P(hat) is greater than or equal to 5.Most common confidence interval selections are 90%, 95%, or 99% but are dependent on the voice of the customer, your company, project, and other factors.

The file also includes the Prism results from the sample data, and these won't change when you add your own data. The larger your sample size, the more confidence one can be that their answers represent the population. I found a section on page 346 in chapter that suggests when using summary statistics, we should round to the same number of places as the sample mean.
If the sample is not then one cannot rely on the confidence intervals calculated, because you can no longer rely on the measures of central tendency and dispersion.Sampling plans are an important step to ensure the data taken within is reflective and meaningful to represent the population.
Click here for information regarding sampling plans.PercentageThe accuracy of the CI also depends on the percentage of your sample that picks a particular answer.
It is easier to be sure of extreme answers than those aren't, thus the interval is not linear.

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