Degrees of freedom for hand graduation

[jensen]

If you have 9 age groups, and you performed graphical graduation on the rates, how many degrees of freedom do you assume for Chi Square test statistic?

Solution 12.19 says 6 or 7, but I thought it should be 8.

 

[Ankmola]

The core reading for graphical graduation states: "A rule of thumb sometimes used is that 2 or 3 DF are lost for every 10 or so ages/ age groups fitted, corresponding roughly to the determination of the height, slope and (perhaps) curvature over that section of the curve."

Even if there are 9 age groups, it should be appropriate to reduce 2 or 3 DFs. I faintly recall a solution set stating "reduce 1 DF each for height, slope and curvature". As curve fitting in a graphical graduation will have to deal with height, slope and curvature, I feel one should reduce 1 DF for each of these i.e. 3 in all.

 

[jensen]

Thanks Ankmola

I realised that after I completed chapter 13. Funny that they have not mentioned anything about it before putting up a question such as this in a chapter that does not touch anything on degrees of freedom for hand graduations.