What is a confidence interval and why it is usefu what is a confidence level?

Answers.com says it is: A statistical range with a specified probability that a given parameter lies within the range. I think that means, just how confident you are that your statistical analysis...

I have always been careless about the use of the terms "significance level" and "confidence level", in the sense of whether I say I am using a 5% significance level or a 5% confidence level in a...

True.

See: http://en.wikipedia.org/wiki/Confidence_interval Includes a worked out example for the confidence interval of the mean of a distribution. In general, confidence intervals are calculated from...

Confidence intervals may be calculated for any statistics, but the most common statistics for which CI's are computed are mean, proportion and standard deviation. I have include a link, which...