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While unit testing a module, it is found that for a set of test data, maximum 90% of the code alone were tested with a probability of success 0.9. The reliability of the module is

1. at least greater than 0.9
2. equal to 0.9
3. at most 0.81
4. at least 1/0.81

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Code tested maximum 0.90%

probability of success is 0.9

So, reliability of the module atmost 0.9 * 0.9 =0.81

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Answer should be atleast or at most how to decide?
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@saumya mishra Since as per qstn , atmost 90% of the code was tested

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@srestha correct 0.90 and not 0.90%
Software Reliability is the probability of failure-free software operation for a specified period of time in a specified environment.
90 % of the code has probability of success as .9.
Let we have 2 blocks one is of 90% of unit and other is of 10%. Let first and second block both has .9 reliability then total reliability is .9*.9=.81.
Since 2nd block will have reliability always less than .9 (given) hence total reliability will be (.9*(no less than .9) hence it will be atmost .81.
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Can't we look it in the below fashion:

With dividing the code in two blocks say 90% and 10 % of code,

total reliability = success of 1st block + success of 2nd block

= 0.9*0.9 + (any success %)*0.1

in any case the total reliability will be atleast 0.81?

Just trying to approach the solution in different way, please comment if I am doing anything wrong.