Re: stucked with chance calculation
- From: daniel_stahl@xxxxxx
- Date: 5 Oct 2005 05:47:49 -0700
Sorry, I think I ll have to clarify my problem to continue this lively
discussion:
The subject has got four choices,
he chooses one item
if this is the correct choice, he stops
if it is the wrong choice, the item will be removed and the
subject is allowed to choose another one (out of 3)
Using Luis approach I - as a not so smart person - would calculate:
P(correct choice in first trial)=0.25
P(coorect choice in second trial)|(first trial wrong choice)
= 1/3 *0.75 =0.25
P(correct)=0.25+0.25 =0.5
So a person who always choses the correct item 5 out of 10 experiments
(chance level for this study design) at the first time is not doing
better than a person choosing the correct item 5 out of 10 times the
second time?
Is this reasoning correct?
Thanks for your help so far,
Daniel
"Luis A. Afonso" wrote:
> In order to start an intelligent discussion about this elementary problem it must be said in fist place that the readers are dealing with two completely distinct situations. I´ll call them problem U and problem W.
>
> ***
> Problem U
>
> The Daniel-Ulrich´s solution is directed to the problem:
> *We perform EXACTLY TWO trials. What is the probability to have TWO successes if the probability of a success in each trial is ½?
>
> Problem W
>
> An ve not too much intelligent person have any difficulty to understand Daniel´s question:
> *..[quoting]...If the person fails to choose the correct one, she/he was allowed to do another go*. (too much complicated to Richard Ulrich!).
>
> My solution is the correct one because
>
> 1) Let A and B be the (radom) events
> _______A success at the first trial
> _______B success only at the second trial.
>
> 2) The two events are mutually exclusive (if one of them occurs the other could not)
>
> 3) They fulfil without any other possibility the event *exactly one success*.
>
> My solution is a direct application of the fundamental probability definition (finite additivity and multiplication law).
>
> _____________licas
.
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