When citing one of these pages, reference the name of the page, the date you accessed it and the URL. All the pages in the statistics and curve fitting guides were written by GraphPad's founder, Harvey Motulsky.
Use this example as a guide to cite a single page:
H. J. Motulsky, "Advice: When to plot SD vs. SEM", GraphPad Statistics Guide. Accessed 5 March 2016. http://www.graphpad.com/guides/prism/10/statistics/index.htm?statwhentoplotsdvssem.htm
Note that the URL for each page in these guides is shown at the bottom of the page. Don't use the URL you see in the browser toolbar, as that may be generic and not apply just to the page you are looking at.
Use this example as a guide to citing the entire statistics guide:
H. J. Motulsky, GraphPad Statistics Guide. Accessed 5 March 2016. http://www.graphpad.com/guides/prism/10/statistics/index.htm