1. Simple Probability

Years since I have done stats, just want to check I have done this correct:

If amazon use 3 companies to deliver parcels, Royal Mail, City Link, Fedex.

Probability of parcel company being used:
Royal Mail = 0.4
Fedex = 0.5

Probability of parcel company delivering to wrong address
Royal Mail = 0.01
Fedex = 0.03

What is the chance of a parcel being delievered to the wrong address?

is it (0.4*0.01) + (0.1*0.02) + (0.5*0.03)?

2. Re: Simple Probability

That is correct. You are essentially using The Law of Total Probability.

3. Re: Simple Probability

Great thanks for confirming that probability of parcel being delivered to wrong address is 0.021

If you know that a parcel has been delivered to the wrong address, to calculate the posterior probability that Royal Mail delivered this, can I just use

P(A/B) = P(AnB) / P (A)

= (0.4 * 0.01 ) / 0.021 = 0.19?

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