"@Home" wrote: > I have estimation of mean / confidence level problem with very litte data > to go on ie no Std Dev, no population infor etc.. > > The unknown is the sample "n" - how large should it be? > The problem is what percentage of apples at a farm have bruises on them? > Only facts are confidence level 95% and error of estimate +/- 4% - Given > this how do you calculate n? > > There is a formula which dispenses w/the need for a standard deviation (they > cancel out), when E is expressed as a fraction of the standard deviation. > ex. > N = (confidence coefficient * standard deviation / E)^2 > N = 1.96 * SD / SD/5 > = 1.96 * 5 > > My book gives examples, but no practical knowledge of how to convert an > error of estimate into a fraction of a standard deviation? > > Thus how would +/- 4% be represented as a fraction of one or more standard > deviations? The number of bruised apples in a barrel (or on a farm) is given to you as a percentage - 10%, 35%, whatever. the number of bruised apples in the barrel is given by (% bruised) * (N = number of apples in the barrel). But I don't think that will answer your problem. If you say that an apple is either (a) bruised, or (b) not bruised, then you are working with a binomial distribution. this can be _approximated_ as a Normal distribution if the total number of bruised apples is greater than 5 (% * N > 5). In case you have not had experience on an apple farm, I can tell you, that in my experience, N is a very large number, and the total number of bruised apples will be much more than 5. So, you can use a Normal distribution assumption. Now, go back to the equation you have used so far, only with the letters still in it, not the numbers. What terms do you have the values for, and which ones do you not have? You certainly don't know n yet. If you know all but n, then you can solve for n, and calculate it out. I think you are still missing something. but I haven't worked it out today, so what do I know? Jay -- Jay Warner Principal Scientist Warner Consulting, Inc. 4444 North Green Bay Road Racine, WI 53404-1216 USA Ph: (262) 634-9100 FAX: (262) 681-1133 email: [EMAIL PROTECTED] web: http://www.a2q.com The A2Q Method (tm) -- What do you want to improve today? ================================================================= Instructions for joining and leaving this list and remarks about the problem of INAPPROPRIATE MESSAGES are available at http://jse.stat.ncsu.edu/ =================================================================
