You are correct there are many reasons why something could have changed and we mark some assumptions to we can can produce some numbers.
I always though that financial analysys gives me questions that I need to answer not solutions
Think of this link
Opening Inventory + Purchases = COGS + Closing inventory
When we are trying to get to cash flow we trying to get Purchases from suppliers. As analaysts we don’t know this (the company does but we don’t tell us) we we use this assumption to help us.
But this assumption may not be correct. Maybe the company toook a huge writedown to inventory that was disclosed a seperate line item and not part of COGS and this does not really work. What about items stolen.
The compay have this detail but we don’t, so we have to make assumpions.
So when trying to calculate cash flow we assume
Opening Inventory + Purchases = COGS + Closing inventory
Purchases = Closing Inventory + COGS - Opening inventory
Purchases = COGS + Change in Inventory
We are making the assumption that any change in inventory can be exaplained by the different in COGS and Purchases. In the normal run of things small amounts of stoken goods, small w/downs breakages will appear in the accounts under COGS so that is ok but there could be issues we need to think about.
The CFA view/approach is simified to what the real world accounting is like. It does though give you the knowledge you need to analysis companies.
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But if we have a write-down on Inventory, shouldn’t that then mean a positive cash flow following the same logic, but it doesn’t right, its a negative cash flow as well?
Remeber if we are think of a w/down is isolation in has no effect on cash it just reduces equity via the income statement, Think were it is most likely to appear in the income statement (assuming it is not large) as part of COGS.
Example
Opening Inv = 100
COGS = 1000
Closing Inv = 50
Purchases = 1000 + 50 - 100 = 950
But what if we realise inventory has a writedown on 20.
But that would have (probably) appeared as part of COGS
Purchases = (980 + 20) + (70 -20) -100 = 950
If the writedown had never happened
Purchases = 980 + 70 - 100 = 950
We make the simplify assumptions when use chnage in inventory to adjust from net income to CFO that the income statement has all the balancing transactions that mean it still makes sense.