This Is Why They Don't Do Joint Performance Reviews

Rajiv and Karthik are 2 traders at the Bank of Hyderabad, who are judged based on their daily Profit and Loss (P&L). Their boss was looking into their performance for the year, and wanted to know whether on a given day, they made money, lost money, or neither made nor lost any money (total of 3 distinct cases).

It was discovered that the probability that Rajiv neither made nor lost any money on a given day is 0.05. The probability that one of them neither made nor lost any money while the other did not make any money is 0.11. The probability that one of them neither made nor lost any money while the other did not lose any money is 0.23.

To 3 decimal places, what is the probability that Karthik neither made nor lost any money on a given day?


The answer is 0.290.

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3 solutions

John Hawksley
Mar 20, 2014

I used a,b,c for Rajiv up/flat/down and x,y,z for Karthik.

We have b = 0.05 and

b(y+z) + yc = 0.11

b(x+y) + ya = 0.23

Simply add the two equations, and then use a+b+c=1

b(x+2y+z) + y(a+c) = 0.34

b(1+y) + y(1-b) = 0.34

y = 0.29

A common mistake made by many people is to use the equation

b ( y + z ) + y ( b + c ) = 0.11 b(y+z) + y(b+c) = 0.11

instead. This double counts the y b yb case.

Calvin Lin Staff - 7 years, 2 months ago

Suppose Probability of Rajiv's profit Rp, Loss Rl, No-profit, no-loss Rn Karthik's profit Kp, loss Kl, no-profit no-loss Kn Rn(Kn+Kl)+KnRl=0.11 Rn(Kp+Kn)+KnRp=0.23 Rp+Rl+Rn=Kp+Kl+Kn=1 solving we get Rn+Kn=0.34 Rn is given as 0.05 So Kn is 0.29

Brendan Wood
Mar 19, 2014

I'll use the following notation:

R u R_{u} is the probability that Rajiv made money on a given day (u for UP)

R f R_{f} is the probability that Rajiv neither made nor lost any money (f for FLAT)

R d R_{d} is the probability that Rajiv lost money (d for DOWN)

Same notation for Karthik ( K u K_{u} , K f K_{f} , K d K_{d} )

since one of those three scenarios must happen each day for each trader, we know that:

1) R u + R f + R d = 1 R_{u}+R_{f}+R_{d} = 1

2) K u + K f + K d = 1 K_{u}+K_{f}+K_{d} = 1

And we are given:

3) R f = 0.05 R_{f} = 0.05

Now we formulate the other probabilities using these variables, assuming that the probabilities of Rajiv and Karthik making money are independent:

4) R f K d + R f K f + R d K f = 0.11 R_{f}K_{d}+R_{f}K_{f}+R_{d}K_{f} = 0.11

5) R f K u + R f K f + R u K f = 0.23 R_{f}K_{u}+R_{f}K_{f}+R_{u}K_{f} = 0.23

We can rewrite 4) as:

R f K d + R f K f + R d K f + R d K d R d K d = 0.11 R_{f}K_{d}+R_{f}K_{f}+R_{d}K_{f}+R_{d}K_{d}-R_{d}K_{d} = 0.11

And factorize:

( R f + R d ) ( K f + K d ) R d K d = 0.11 (R_{f}+R_{d})(K_{f}+K_{d})-R_{d}K_{d} = 0.11

Similarly for 5):

( R f + R u ) ( K f + K u ) R u K u = 0.23 (R_{f}+R_{u})(K_{f}+K_{u})-R_{u}K_{u} = 0.23

Substituting in for R f R_{f} and K f K_{f} from equations 1) and 2) yields:

( 1 R u ) ( 1 K u ) R d K d = 0.11 (1-R_{u})(1-K_{u})-R_{d}K_{d} = 0.11

( 1 R d ) ( 1 K d ) R u K u = 0.23 (1-R_{d})(1-K_{d})-R_{u}K_{u} = 0.23

Expanding we have:

1 R u K u + R u K u R d K d = 0.11 1-R_{u}-K_{u}+R_{u}K_{u}-R_{d}K_{d} = 0.11

1 R d K d + R d K d R u K u = 0.23 1-R_{d}-K_{d}+R_{d}K_{d}-R_{u}K_{u} = 0.23

Adding these two equations and simplifying gives:

1 R u K u + R u K u R d K d + 1 R d K d + R d K d R u K u = 0.11 + 0.23 1-R_{u}-K_{u}+R_{u}K_{u}-R_{d}K_{d}+1-R_{d}-K_{d}+R_{d}K_{d}-R_{u}K_{u} = 0.11+0.23

( 1 R u R d ) + ( 1 K u K d ) = 0.34 (1-R_{u}-R_{d})+(1-K_{u}-K_{d}) = 0.34

R f + K f = 0.34 R_{f}+K_{f} = 0.34

Substituting in the given value for R f R_{f} (equation 3) leads us to conclude:

K f = 0.29 \boxed{K_{f} = 0.29}

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