standard-deviation

I am trying to use the StDev function and am getting blank results. I am using it as... SELECT StDev(fldMean) FROM myTable Where fldMean contains a value of 2.3 and should evaluate to 0 but instead I am simply getting an empty result. I can't seem to understand how expressions are to be used in the function, Microsoft's manual really didn't help. SELECT StDev(fldMean) FROM myTable will return Null if [myTable] has only one row because the Standard Deviation cannot be computed from a single observation. You will need at least two rows in that table before you can receive a meaningful result. If

I have data with discrete x-values, such as x = c(3,8,13,8,13,3,3,8,13,8,3,8,8,13,8,13,8,3,3,8,13,8,13,3,3) y = c(4,5,4,6,7,20,1,4,6,2,6,8,2,6,7,3,2,5,7,3,2,5,7,3,2); How can I generate a new dataset of x and y values where I eliminate pairs of values where the y-value is 2 standard deviations above the mean for that bin. For example, in the x=3 bin, 20 is more than 2 SDs above the mean, so that data point should be removed. for me you want something like : by(dat,dat$x, function(z) z$y[z$y < 2*sd(z$y)]) dat$x: 3 [1] 4 1 6 5 7 3 2 ---------------------------------------------------------------

I'm trying to detect people using proxies to abuse my website. Often they will change proxies and so forth. But there is definitely a pattern of them using one proxy address many times. Much more than is normal for legitimate visitors. Usually most accessing of my website is by unique ip addresses that have only visited once or a few times. Not repeatedly. Let's say I have these ip addresses in a column: 89.46.74.56 89.46.74.56 89.46.74.56 91.14.37.249 104.233.103.6 That would mean there are 3 uniques out of 5. Giving a "uniqueness score" of 60%. How would I calculate this efficiently using

If I have a signal as the one below, how would I go about finding the beginning and end of the two "major events" (illustrated by a green arrow where the event begins, and a red arrow where it ends)? I've tried the method suggested in this answer , but it seems that no matter how much I play around with the lag , threshold and influence variables, it either reacts to the tiny changes in the beginning, middle and end of the graph (where there are no major events), or it doesn't react at all. I can't simply determine if the signal is above a fixed threshold, as the strength of the signal can