I have to collect a lot of data for an entity.
I have decided to split the table up into sub-tables and form relationships all with the same primary key to the parent. Is it better to do this , or should I try and store as much data as possible in as few tables as possible. My only concern is that some tables may have 20 or so columns if I use few big tables. is that OK?

20 columns sounds pretty small to me - unless you are using a "toy" DBMS? What DBMS are you using?

SQL Server 2005 (not express). Just concerned about managing them afterwards really.

SQL Server should handle 20 columns very comfortably I'm sure. I wouldn't consider vertical partitioning (which is what you are proposing) merely to reduce the number of columns of a 20 column table - a 200 column table maybe.

Sounds like a good idea. At the moment, my diagram looks like a bit of a birds nest. I'm going to reduce the number of tables. Thanks for the advice!

To add to that, would I be correct in saying that any one-to-one relationships I have with the parent can be removed and those attributes from the child table placed in the parent table and the child table could be discarded?

To add to that, would I be correct in saying that any one-to-one relationships I have with the parent can be removed and those attributes from the child table placed in the parent table and the child table could be discarded?

I currently have 51 tables for this entity

They could, if you wish. OTOH, if there is a child table that has many columns and most parent rows don't have such a child, then you may prefer to keep all those rarely-used columns out of the parent table. But I'd err in favour of the simpler approach (one table, may columns) unless there is a compelling reason (e.g. poor performance) to do otherwise - and unless you can verify that splitting into more tables reduces that problem.

This is very much a trade-off between simplicity on the one hand and performance on the other - there is no one right answer.

Awesome. Thanks a lot for your help. This dbforum is a really good source of info.