Approach:

1. Follow up questions to understand the scope

2. Formula to estimate the apps count

3. Conclude

Follow up questions:

Candidate: Are we interested in US or WW?

Interviewer: WW

Candidate: Do we want focus only on the apps that are actively supported by the developers or all apps ever created

Interviewer: All apps

Candidate: Do want to accont for apps that are later removed from the app store?

Interviewer: No

Formula to estimate the app count

Step 1: Estimate the number of app created in 2008 when app store was launched

Let us assume in 2008 there were 500K app developers in the world. Ten developers creates one app in a year. That’s 50K apps.

Step 2: Assume that 10% more apps are created in each following year

2008	50,000.00
2009	55,000.00
2010	60,500.00
2011	66,550.00
2012	73,205.00
2013	80,525.50
2014	88,578.05
2015	97,435.86
2016	107,179.44
2017	117,897.38
2018	129,687.12
2019	142,655.84
2020	156,921.42
2021	172,613.56

Step 3: Sum all the apps that were creaed over 12 years; it comes about 1.4MM apps in total ever created.

My advice on answering this estimation question in a Google PM interview:

Clarifying Questions

App store contains generally 4 catrgories – Games, Apps, Books, Movies, Audios (Songs & Podcasts). Are we focusing on only apps or are we considering games are apps too.

Solution

Considering app stores host many apps, these apps are divided into various categories. Top categories are listed below.

Considering there are almost ~200  countries in the world and almost 40% population is using smarphone now. We need to create an equation for each categroy listed below

Total Coutries = 200

Countries with Internet / Smartphone infrastructure as well as users = 150

Hence for a given number of categories below. we will have country specific variants of simiar apps.

Example – Uber in USA while OLA in India and DIDI in China for ride hailing.

Additionally, these apps overlap geography as well. hence we will list down unique apps ideas below and take n% of them as universal apps while 100-n% being replicated in all countries with localization, culture etc.

 

1. Camera / Photography apps

• Total unique ideas = 2000
• Universal app = 20% = 400
• Country Specific Variants = 1600 * 150 = 240,000
• Total = 240,400

2. Social Media and Communication Apps

• Total unique ideas = 1000
• Universal app = 20% = 200
• Country Specific Variants = 800 * 150 = 120,000
• Total = 120,200

3. News Apps

• Total unique ideas = 500
• Universal app = 20% = 100
• Country Specific Variants = 400 * 150 = 60,000
• Total = 60,100

4. Exercise Apps

• Total unique ideas = 400
• Universal app = 50% = 200
• Country Specific Variants = 200 * 150 = 30,000
• Total = 30,200

5. Productivity Apps

• Total unique ideas = 500
• Universal app = 80% = 400
• Country Specific Variants = 100 * 150 = 15,000
• Total = 15,400

6. Utility Apps – Electricity / Water / IOT / Food delivery etc

• Total unique ideas = 100
• Universal app = 0% = 0
• Country Specific Variants = 100 * 150 = 15,000
• Total = 15,000

7. Finance and Banking related apps

• Total unique ideas = 500
• Universal app = 20% = 100
• Country Specific Variants = 400 * 150 = 60,000
• Total = 60,100

8. OTT Streaming apps

• Total unique ideas = 400
• Universal app = 50% = 200
• Country Specific Variants = 200 * 150 = 30,000
• Total = 30,200

9. Video Calls Apps

• Total unique ideas = 100
• Universal app = 0% = 0
• Country Specific Variants = 100 * 150 = 15,000
• Total = 15,000

10. Transport related apps

• Total unique ideas = 100
• Universal app = 0% = 0
• Country Specific Variants = 100 * 150 = 15,000
• Total = 15,000

11. Travel Apps

• Total unique ideas = 500
• Universal app = 10% = 50
• Country Specific Variants = 450 * 150 = 67,500
• Total = 67550

12. Fashion Apps

• Total unique ideas = 400
• Universal app = 50% = 200
• Country Specific Variants = 200 * 150 = 30,000
• Total = 30,200

13. E-Commerce Apps

• Total unique ideas = 100
• Universal app = 0% = 0
• Country Specific Variants = 100 * 150 = 15,000
• Total = 15,000

14. Miscellaneous

• Total unique ideas = 10000
• Universal app = 0% = 0
• Country Specific Variants = 1000 * 150 = 1500,000
• Total = 1500,000