Whats the formula for variance between batches?

[Fat Ryan]

Im working on a lab for my Civil Engineering Materials class and its asking for the "between batch variation" (SBB^2) for different cylinders of Portland cement concrete. Its not written anywhere on the lab sheet. And Ive looked in my book, notes, and Google. Im getting nothing here :/

[Fat Ryan]

any1?

[hhn2002]

keep in mind im not studying to be a civil but mech engineer so this is the best i can come up with.

heres a link to a digital book that discusses batch variations:
http://books.google.com/books?id=g8RH7Feop-QC&pg=PA91&lpg=PA91&dq=cement+between+batch+variation&source=bl&ots=SdkKVA7der&sig=N_1b9hJdtNfTBH--X4Ua631M2-Q&hl=en&ei=fYOuSo6NNcultge9msSTCA&sa=X&oi=book_result&ct=result&resnum=1#v=onepage&q=cement%20between%20batch%20variation&f=false

if you look at table 1.7 and read the page above it it seems as if S=compressive strength in MPa. so you are looking at the difference between compressive strength of both batches.
absolute value of S(batch A)-S(batch B).
maybe your making it harder than it seems when it might just be a trivial subtraction operation. and maybe that is why you cant find a formula to it because its that simple.
anyways keep in mind this is not my specialty and i know absolutely nothing about civil but this is what i understood purely from the text.

[brewnog]

If you really expect help within 35 minutes of posting your question, you're in the wrong place.

[Fat Ryan]

hhn, thanx for the help. i ended up getting help from another grad student before i could even get back to my computer though. thanx for your time though.