Hello Beauty!
Did you find the answer of your question? I have the same question and wondering if can provide me the answer. Thank you, H 
There is no sampling frequency option. There is only a sampling interval.
Sampling Interval = 1 means sample every timestep Sampling Interval = 2 means sample every other timestep To achieve a certain sampling frequency (with units 1/s), you must choose the sampling interval and timestep size accordingly. The relation between frequency, sampling interval, and timestep size given was correct. frequency = 1/(stepsize x sampling interval) 
Thank you LuckyTran!

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sample interval = 200 means it will calculate average value of all 200 time steps. is it right? i am using delta_t = 10^6, could you please suggest me what sampling interval should i use? thank you 
It means it will collect data each 200 time steps. Hence, after the first 200 time steps, your statistics will coincide with the instantaneous field saved at the 200th time steps.
To do what you wrote you need sample interval = 1. This, btw, is always your best choice if you don't know what you're doing. 
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if i select select sample interval =1 or 10 then i should save Data and case file for every 1 or 10 time step respectively. if sample interval is 3 and autosave of data and case file is 100 then i wont get u', v' and w' at any time step. am i right? if not then please explain. thank you 
I never tried but i suspect that it simply works such that when you save you just get the last set of collected samples. For example:
Autosave each 100 time steps Sample interval 30 time steps Then, i guess, at the first autosave after 100 dt the statistics saved in your file are relative to: Time 0 (not sure here) Time 0 + 30 dt Time 0 + 60 dt Time 0 + 90 dt Still, obviously, i can't see any reason to do something like this. In theory, the sample interval should be the minimum to achieve sufficient statistical independence between the samples. In practice, 1 works just great if you collect enough samples. 
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