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From |
"Nick Cox" <n.j.cox@durham.ac.uk> |

To |
<statalist@hsphsun2.harvard.edu> |

Subject |
RE: st: cumulative average moving through time |

Date |
Wed, 6 Oct 2004 21:28:50 +0100 |

Be very careful here. You're confusing some quite different beasts. -egen- ====== -egen, sum()- fires up an -egen- function which produces totals. Under -by:- or with a -by()- option it produces group totals. You can find the code in -_gsum.ado- (-which _gsum- will find where on your machine). In essence, -egen- only takes -egen- functions, either as documented under -[R] egen-, or as user-defined -egen- functions _always_ flagged as such. Also, -egen- functions are _never, ever_ allowed anywhere else. They require -egen- absolutely. -egen- is really rather limited. There are perhaps of the order of 100 -egen- functions written, and that's a fixed menu, except insofar as if you don't like them, you can indeed write your own. -sum()- and other functions =========================== -sum()- anywhere else it is legal fires up the -sum()- function which produces cumulative sums. This is part of the executable and has been so for a very long time, perhaps even since Stata 1.0. -generate- (and -replace-) can in effect take very complicated expressions as arguments, making use of constants, variables, operators and functions such as -sum()-. The scope of -generate- is in no way indicated by the few token examples in the help. By combining constants, variables, operators and functions, you have _much_ more flexibility than with -egen-. Why then bother with -egen-? Just for convenience, that some often repeated sets of operations have been rolled into -egen- functions. Name conflict! ============== If you find this confusing, or difficult to defend, you are in excellent company. Svend Juul gave a very droll paper at the Berlin users' meeting in which he underlined this and a few other messes over names. StataCorp are known to be taking the issue seriously. At the same time, the last thing they want to do is to break any existing programs, do files or habits. by: === One source of explanations is How to move step by: step. Stata Journal 2(1): 86-102 (2002) which gathers the main ideas in one place. The obvious alternative is to look up -by- in the Manual index and read the several sections thus indicated. The article just mentioned was written because the coverage of -by:- in the manuals is a bit fragmented. It's been said by a long-time Stata user that wrapping your head around the possibilities of -by:- is the biggest single step you can take to real Stata fluency. Nick n.j.cox@durham.ac.uk Daniel Egan > > by sort pid (ob):gen cave = sum(calc)/ob > > This is so obvious as to be painful. So why didn't I think of it? > > 1) Where/When did -sum()- become an acceptable argument to > -generate-!?!? I have only ever seen it in the context of -egen-. > Looking at the help for -generate-, there are no arguments that are > explicitly stated to be useable. It is only at the very bottom of the > examples that one sees an function -uniform- and then -sum- used with > gen. Are the others? I know that using many egen arguments with -gen- > will return errors (e.g. count). > > 2) Why does the ---bys pid (ob)-- do this correctly? I understand that > it is equivalet to --sort PID OB--, but why does it result in the > correct cumulative sum? > Another way of putting this is why doesnt -egen cave=sum(calc)/ob, > by(PID OB)- work if this does? > * * For searches and help try: * http://www.stata.com/support/faqs/res/findit.html * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

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