“If in doubt, do half” is an investment adage that urges reducing risk. “Whilst on the subject of Cottle, one of his catch phrases is ‘If in doubt, do half’ i.e. close or adjust half your position of you aren’t sure what is going to happen” was cited in 2006. The reference is to Charles Cottle (RiskDoctor.com).
 
Alan Clarke of Alan Clarke Financial Services has popularized the adage in New Zealand.
       
 
EliteTrader
Jan 17, 2006
momoneythansens
Whilst on the subject of Cottle, one of his catch phrases is “If in doubt, do half” i.e. close or adjust half your position of you aren’t sure what is going to happen. Some folk seem to get stuck in the “all or nothing” mindset and it’s useful to remind oneself that this is not necessary.
   
Alan Clarke Financial Services Ltd. (New Zealand)
Invest, don’t gamble
10/15/2013
(...)
If in doubt, do half
This is one of the best rules I know. It can apply to buying or selling just about any investment,  to loans to children, helping a wayward brother, and so on.
 
Alan Clarke Financial Services Ltd. (New Zealand)
If in doubt, do half
5/13/2014
This is a great rule, and is not used nearly enough, as it can be applied to so many issues.
 
The Aucklander (New Zealand)
Alan Clarke: Bottoms up to bottoming out
By Alan Clarke
9:00 AM Wednesday Oct 1, 2014
(...)
Fear can be overcome by working the old adage: “If in doubt, do half”.
 
Spend or invest only half your available cash, then wait for six months and reappraise.