Basic WAIS IV question

[urmaserendipity85]

I should know this...
If someone has a variable WAIS IV profile, and two of their indices different, is it one or two standard deviations where would be starting to suggest the FSIQ is not a valid estimate of their general intellectual abilties?
The scores I have in front of me are in the mid 70s for VCI, PRI and PSI, but in the 50s for WMI. The difference between the highest index score (VCI) and WMI is 24 points. I realise that this is a big difference and therefore the FSIQ in the late 60s may not be representative of his abilities, but I'm talking an AP through the scoring and just want to tell her the right thing in terms of we start to dismiss the validity of the FSIQ when scores are x points apart.
I don't think I'm articulating myself very well, and I definitely did know this stuff at one point, but it's a friday afternoon and I'm blonde
Anyone else think life was simpler when we just had the VIQ and PIQ to worry about?
TVM x

[Nephthys]

I believe that it's a difference of 21 points when the FSIQ would be considered invalid but this is just going from memory. It isn't in the manual but it's definitely in the Essentials of WAIS-IV book if you happen to have it handy in your office. Otherwise, if someone else happens to have it handy, perhaps they could check for you?

[urmaserendipity85]

Found the answer, it's 1.5 SDs which is 23 points. Thanks