Here's a graph of some data:
How do I tell if the blue slope is significantly different from either of the grey slopes? I know just enough about statistics to know that there will be a way to test this, but not enough to do it. And the post-doc I've relied on for statistics help just moved on to a second post-doc position on the other side of the country. What to do?
Google the problem, of course. I think I searched with 'calculate confidence interval for slope, and that led me to this page, one of a collection of Excel Tips for scientists and engineers posted by one Bernard Liengme, a retired professor of chemistry and lecturer in information systems at St. Francis Xavier University in Nova Scotia.
At first the page looked quite daunting. The post-doc confirmed by email that this was instructions for doing what I wanted, but didn't offer to do it for me. I then tried clicking on the link to the sample workbook, which gave me an actual Excel file with the example calculation all set up. So I just did to my data what the example did to its, and presto, I have the confidence intervals for my lines! it's a bit embarrasing to admit that I don't know what the "INDEX(LINEST" command does, but then I don't know what's in the secret buffers and columns of the kits we use either.
So thank you Dr. Liengme! Note - a new edition of his book A Guide to Microsoft Excel 2007 for Scientists and Engineers is available in paperback from Amazon.
p.s. The 95% confidence interval for the blue line overlaps slightly the intervals for the grey lines.
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