it's probably also worth mentioning that carb ice can form on warm days as well as cool days, due to the potential size of the temperature drop.
The other key factor is relative humidity, so you may be free of carb ice on a dry, cold day, but plagued with it on a warm day, with a high dewpoint.
So an aircraft that experiences no carb ice on one day at a given power setting, may experience icing on a different day. That's one of the reasons that pilots flying in humid areas tend to apply carb heat fairly regularly as they cruise and certainly before reducing power.
Some aircraft/engine combinations seem more prone to ice, from personal experience the C150/Continental 0-200 needs careful monitoring, whereas the PA28/Lycoming 0-360 seems to be more resistant.
However, the bottom line is that you can get carb ice on any engine and it can form quickly, so awareness and monitoring is the safest approach.
For more info, you maybe interested in following this link
http://www.caa.co.uk/docs/33/SRG_GAD_SSL14.PDF