I'm not talking about glass balls and Mystic Meg but tools and modellers that are relied on for predicting forward our budgets, cash flow, pensions and investments will all use a series of assumptions for example inflation, annuity rates, investment growth etc. 

One of the most fundamental truths when planning and investing is to read the small print. This also applies when using modellers; check the assumptions on which they are based and make sure those assumptions are in line with your circumstances and needs.

Another important truth when planning finances is that tools and modellers should be used as an indicator only and they need to be regularly reviewed, especially if there are significant changes to  key measures such as inflation or investment performance.

Sometimes, such as with the government's student loan calculator, the assumptions will be unsuitable for a group of people - here for all female students.

Modellers are a fantastic tool for financial planning but they are worthless or worse still could be misleading if the assumptions on which they are based don't reflect your circumstances, needs and wishes.