This looks like a good place to ask:

I am using a spreadsheet to organize 15 days' worth of stock market price
data.

Each day of data (row: symbol in text, low price, high price),
in column form, is a bit different from every other days' column of
data, because a handful of stocks are added or dropped on an almost
daily basis. Each day has more than 7000 stocks.

Problem: how do I sort out and discard all the stocks in a column that do NOT
appear in each and every of the other 14 days?

Though MS Works Spreadsheet is grossly inadequate for the task, there
is a way to do it (but it's time-consuming, involving a lot of cutting
and pasting and SORT and a choice formula or two).

Some shareware spreadsheets have COUNTIF, but I can't get the COUNTIF
to reference text (symbol) from another cell for its value. (Method:
count each symbol's occurence in the entire 15 days' range of data.
Discard every stock that occurs less than 15 times.)

Any ideas?