Hi,

First of all - yes you should **always tell your approach first.**

Secondly - **never say - let's assume 50%.** Your assumption should always be based on something:

**Personal experience** (e.g. in my apartment building with 100 households, I see 10 people walking with the dogs every morning. So I would assume that 10% households have a dog)
**General knowledge** (e.g. The average family size is 3 people)

You should also** adjust your assumption** where it's possible (E.g. The average family size is 3 people, but since we are talking about New York where people are mostly single, I would assume the av. household size of 2 people)

Here are some useful market sizing skills:

1) First of all, there are **2 ways to structure market sizing**:

- Formula - basically a math formula to come up with a solution. The problem with the formula is that it is easy to forget something or get lost.
- Tree - same as with regular cases you build a tree. A very simple example: you need to calculate the number of dogs on manhattan. A number of dogs = share of households having a dog * # of households. # of households = population / average household size. In the end, you'll have a pyramid where you have to fill the numbers on the base of the pyramid. This approach is much easier and help you track all the numbers

2) You should **learn the key market sizing techniques:**

- Making assumptions based on personal experiences (Use the example of your house where out of 100 apt-s 10 have dogs)
- Adjusting numbers (NY is a busy city thus fewer people have dogs)
- Sanity check - try to apply your calculations to real environment
- etc.

3) You should **learn the key tools:**

- Using age even age split (suppose life expectancy is 80 years. Assuming even age split we have 4 mln people in US of each age group)
- Using 80/20 split (suppose 20% people earn 80% wealth and the average salary is xx...)
- Using approximations (Length of NY-SF flight and plane speed to calculate US length)
- etc.

4) Learn** key numbers**: populations, gas price, gas consumption, Boeing speed and nmber of seats, average salary, # of gates in the airport, GDP growth rate, inflation, etc.

5) **Practice 10-15 cases **and you'll be fine

Feel free to PM for clarifications

Good Luck!