How financial analysts can predict a forecast for macro indicators like CPI, GDP?

Hi.. Does anybody know the answer?

For example CPI is a measure estimating the average price of consumer goods and services purchased by households which means that from time to time national statistical agencies send theirs price agents into the streets and they collect actual prices of items from market basket. And Market basket contains many many exotic items.

well ok.. this was a theory

no how financial analysts can predict CPI(for example). Do they use percentage changes from other macro indicators or something like that?

I can't believe that analysts are creating their own market basket and send agents for prices

so does anyone know where is the truth?

I'd say this might be one scenario maybe, next months forecast might be 3.2%, but say in two weeks time, something happens and petrol goes up 10% and you notice that your shopping is another £15 or something, it would be a fair bet that it would be higher than 3.2%.

Of course you can always go into more detail and more complex calculations, but I always argue that the volatility of the market is always going to be more than your detailed calculation.