Author Topic: CSI and cadaver dogs - some facts and statistics.  (Read 69362 times)

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Offline Mr Gray

Re: CSI and cadaver dogs - some facts and statistics.
« Reply #45 on: May 06, 2015, 08:28:07 AM »
Yes, I have a degree in it which is why I am questioning you arbitrary selection of the probability you want to combine.

a degree in statistics...are you sure...no such thing

Offline ShiningInLuz

Re: CSI and cadaver dogs - some facts and statistics.
« Reply #46 on: May 06, 2015, 08:31:43 AM »
If you have a degree in statistics you should understand the argument above; anyone can claim anything on here and I do not accept your expertise. Try working it through from first principles.

Two blood dogs each with an average of 80% reliability.

Parallel Case

Dog one alerts giving one reason to believe that there is the possibility of 4 in 5 cases being correct.

Dog two alerts giving one reason to believe that there is a possibility of 4 in 5 cases being correct.

As you are repeating the same test, the second test increases the accuracy and two alerts increase the chance of the dogs being correct.

Serial Case

Dog one alerts giving one reason to believe that there is the possibility of 4 in 5 cases being correct.

Dog two alerts giving one reason to believe that there is a possibility of 4 in 5 cases being correct.

As the two rests are not connected the second test introduces inaccuracy and the accuracy is decreased.
So if only we could wipe out one dog's response from the record, we would be left with a more accurate record?  Shame on Grime for using two dogs and thus reducing the probability of clear evidence.  Or perhaps he is to be praised for not using 3, that really would have been a mess!

Sorry, but I'm with Slarti on this one.  It's the error rates that are important, not the success rates.  And before you respond, yes, I've studied applied stats at uni level, though that was not my degree subject.

Slarti 1 Bloo 0.

And since we don't know the dogs success or error rates, it is all irrelevant!
What's up, old man?

Offline Mr Gray

Re: CSI and cadaver dogs - some facts and statistics.
« Reply #47 on: May 06, 2015, 08:35:37 AM »
So if only we could wipe out one dog's response from the record, we would be left with a more accurate record?  Shame on Grime for using two dogs and thus reducing the probability of clear evidence.  Or perhaps he is to be praised for not using 3, that really would have been a mess!

Sorry, but I'm with Slarti on this one.  It's the error rates that are important, not the success rates.  And before you respond, yes, I've studied applied stats at uni level, though that was not my degree subject.

Slarti 1 Bloo 0.

And since we don't know the dogs success or error rates, it is all irrelevant!

If Grime wanted the alerts taken seriously he would need to have the dogs independently  tested and verified.He hasn't because what is important is not the alerts but the evidence the dogs find. the alerts themselves are of no significance

OxfordBloo

  • Guest
Re: CSI and cadaver dogs - some facts and statistics.
« Reply #48 on: May 06, 2015, 08:51:38 AM »
So if only we could wipe out one dog's response from the record, we would be left with a more accurate record?  Shame on Grime for using two dogs and thus reducing the probability of clear evidence.  Or perhaps he is to be praised for not using 3, that really would have been a mess!

Sorry, but I'm with Slarti on this one.  It's the error rates that are important, not the success rates.  And before you respond, yes, I've studied applied stats at uni level, though that was not my degree subject.

Slarti 1 Bloo 0.

And since we don't know the dogs success or error rates, it is all irrelevant!

You misunderstand completely.

OxfordBloo

  • Guest
Re: CSI and cadaver dogs - some facts and statistics.
« Reply #49 on: May 06, 2015, 10:17:59 AM »
Yes, I have a degree in it which is why I am questioning you arbitrary selection of the probability you want to combine.


You have produced no reasoning but are merely denying from a position of claiming you simply know because you studied the subject-an argument from authority.

I have explained the reasoning in a way that any person understanding statistics should understand. I will now resort to Statistics 101 to explain why such parallel observations reduce the accuracy by the square or product of the first uncertainties.

Assume the is cadaver odour present and five tests are run with each dog.
Eddie alerts four times but gives a false negative once. 80% correct.
Keela does not alert four times but gives a false positive once. 80% correct.

Each second trial needs to be distributed over each original trial

Eddie       T   T   T   T   F
Keela   T   P   P   P   P   N
   T   P   P   P   P   N
   T   P   P   P   P   N
   T   P   P   P   P   N
        F   N   N   N   N   N

Proportion of correct calls is 16 out of 25

Or 64%.


Offline VIXTE

Re: CSI and cadaver dogs - some facts and statistics.
« Reply #50 on: May 06, 2015, 10:23:36 AM »

You have produced no reasoning but are merely denying from a position of claiming you simply know because you studied the subject-an argument from authority.

I have explained the reasoning in a way that any person understanding statistics should understand. I will now resort to Statistics 101 to explain why such parallel observations reduce the accuracy by the square or product of the first uncertainties.

Assume the is cadaver odour present and five tests are run with each dog.
Eddie alerts four times but gives a false negative once. 80% correct.
Keela does not alert four times but gives a false positive once. 80% correct.

Each second trial needs to be distributed over each original trial

Eddie       T   T   T   T   F
Keela   T   P   P   P   P   N
   T   P   P   P   P   N
   T   P   P   P   P   N
   T   P   P   P   P   N
        F   N   N   N   N   N

Proportion of correct calls is 16 out of 25

Or 64%.

Hmmm.. then if we analyse Portuguese GNR dogs, their percentage of showing 'alive' Madeleine leaving the apartment on foot was higher. And also more significant, because they were brought to the crime scene only hours later!

Or is it politically correct to only trust Keela and Eddie i.e the presentation of their alerts? And if yes then why?

That much about statistics hahaaha

Offline Alice Purjorick

Re: CSI and cadaver dogs - some facts and statistics.
« Reply #51 on: May 06, 2015, 10:29:17 AM »
Still playing set pieces I see.
"Navigating the difference between weird but normal grief and truly suspicious behaviour is the key for any detective worth his salt.". ….Sarah Bailey

OxfordBloo

  • Guest
Re: CSI and cadaver dogs - some facts and statistics.
« Reply #52 on: May 06, 2015, 10:40:39 AM »
Hmmm.. then if we analyse Portuguese GNR dogs, their percentage of showing 'alive' Madeleine leaving the apartment on foot was higher. And also more significant, because they were brought to the crime scene only hours later!

Or is it politically correct to only trust Keela and Eddie i.e the presentation of their alerts? And if yes then why?

That much about statistics hahaaha

That is nothing to do with the reasoning here which shows how the uncertainties multiply and reduce in such trials.

Offline slartibartfast

Re: CSI and cadaver dogs - some facts and statistics.
« Reply #53 on: May 06, 2015, 12:58:08 PM »

You have produced no reasoning but are merely denying from a position of claiming you simply know because you studied the subject-an argument from authority.

I have explained the reasoning in a way that any person understanding statistics should understand. I will now resort to Statistics 101 to explain why such parallel observations reduce the accuracy by the square or product of the first uncertainties.

Assume the is cadaver odour present and five tests are run with each dog.
Eddie alerts four times but gives a false negative once. 80% correct.
Keela does not alert four times but gives a false positive once. 80% correct.

Each second trial needs to be distributed over each original trial

Eddie       T   T   T   T   F
Keela   T   P   P   P   P   N
   T   P   P   P   P   N
   T   P   P   P   P   N
   T   P   P   P   P   N
        F   N   N   N   N   N

Proportion of correct calls is 16 out of 25

Or 64%.

Changing the goal posts I see. We were talking about cadaver dog accuracy. In your example there is only one case were both dogs are wrong I.e. 1/25 =4%.

In multiple tests for cadaver the more tests are made the less likely are the alerts to be incorrect.
“Reasoning will never make a Man correct an ill Opinion, which by Reasoning he never acquired”.

OxfordBloo

  • Guest
Re: CSI and cadaver dogs - some facts and statistics.
« Reply #54 on: May 06, 2015, 01:11:45 PM »
Changing the goal posts I see. We were talking about cadaver dog accuracy. In your example there is only one case were both dogs are wrong I.e. 1/25 =4%.

In multiple tests for cadaver the more tests are made the less likely are the alerts to be incorrect.

The above is amended from a statistics text demonstrating why uncertainty in drug trials decreases rather than increases if the final outcome is determined by two separate tests which are unconnected and different in kind.

The matrix above demonstrates every possible outcome for two dogs, one cadaver/blood, one blood alone, reacting to the possible of presence of cadaver- exactly what we are arguing about. In twenty five possible results, sixteen are correct and nine are wrong, meaning that sixteen out of twenty five are right which is 64%.

Please supply your reasoning in a similar way to demonstrate that the certainty increases rather than decreases.

If you cannot we shall just have to assume you have forgotten the statistical ability you claim to have had once.

No simple denial please- rational argument with mathematical rigidity.

OxfordBloo

  • Guest
Re: CSI and cadaver dogs - some facts and statistics.
« Reply #55 on: May 06, 2015, 01:18:21 PM »

In multiple tests for cadaver the more tests are made the less likely are the alerts to be incorrect.

A hint to help. Your last statement is why you are getting it wrong. Multiple tests by cadaver dogs tell you nothing about the presence of cadaver as they result in a finding of 'blood or cadaver'. The initial result needs to be decided by a negative given by a blood dog.

I will agree that if you introduced a second team and doubled the alerts and both pairs agreed, THAT would increase the probability of accuracy, but in the case in point, the requirement for two separate tests introduces extra uncertainty by the product of the original uncertainties.

Think about it rather than just knee jerking and if you still disagree, produce similar reasoning for what you claim. Simple contradiction from a claim to authority just does not pass muster.

Offline Alice Purjorick

Re: CSI and cadaver dogs - some facts and statistics.
« Reply #56 on: May 06, 2015, 01:24:39 PM »
A hint to help. Your last statement is why you are getting it wrong. Multiple tests by cadaver dogs tell you nothing about the presence of cadaver as they result in a finding of 'blood or cadaver'. The initial result needs to be decided by a negative given by a blood dog.

I will agree that if you introduced a second team and doubled the alerts and both pairs agreed, THAT would increase the probability of accuracy, but in the case in point, the requirement for two separate tests introduces extra uncertainty by the product of the original uncertainties.

Think about it rather than just knee jerking and if you still disagree, produce similar reasoning for what you claim. Simple contradiction from a claim to authority just does not pass muster.

So what is the objective of this set piece?
"Navigating the difference between weird but normal grief and truly suspicious behaviour is the key for any detective worth his salt.". ….Sarah Bailey

OxfordBloo

  • Guest
Re: CSI and cadaver dogs - some facts and statistics.
« Reply #57 on: May 06, 2015, 01:43:53 PM »
OK let us ignore dogs.

Take a bag of pebbles that has eighty per cent red spheres and twenty per cent blue spheres.
Picker one draws five balls on average they will draw four them in the ratio 4:1 and can predict that the bag contains balls in a ratio of four to one with an uncertainty of p
If picker two repeats the same exercise then the uncertainty would fall to p + x – it is more likely to be right.

Now consider a bag containing balls which has eighty per cent hollow spheres and twenty per cent solid spheres.
Picker one draws five balls on average they will draw them in the ratio of 4:1 and can predict that the bag contains balls in a ratio of four to one with an uncertainty of p
If picker two repeats the same exercise then the uncertainty would fall to p + x – it is more likely to be right.


Now consider a bag which includes spheres each of which is either red or blue and independently each is hollow or solid.
Balls are withdrawn by a neutral party.
Picker one  only sees the balls and determines their ratio of red to blue balls.
Picker two who is blind only weighs the balls and determines whether the balls are solid or hollow.

Now their probability decreases according to the same matrix

   T   T   T   T   F
T   P   P   P   P   N
T   P   P   P   P   N
T   P   P   P   P   N
T   P   P   P   P   N
F   N   N   N   N   N

Proportion of correct calls is 16 out of 25


Lyall

  • Guest
Re: CSI and cadaver dogs - some facts and statistics.
« Reply #58 on: May 06, 2015, 01:52:39 PM »
If Grime wanted the alerts taken seriously he would need to have the dogs independently  tested and verified.He hasn't because what is important is not the alerts but the evidence the dogs find. the alerts themselves are of no significance

You're right, on their own they aren't. But when combined with the evidence of the training of the dog they are.

Offline John

Re: CSI and cadaver dogs - some facts and statistics.
« Reply #59 on: May 06, 2015, 02:33:21 PM »
I must have missed that announcement by Grange. Could you provide a cite for it.

Of course they may not have been searching for Madeleine, but seeking to exclude possibilities and thus increase the likelihood that she was not dead and buried in PdL.

What?   You missed all the media reporting on where, when and why they were digging up parts of Praia da Luz?    Do you honestly believe they weren't looking for remains?

« Last Edit: May 06, 2015, 02:50:46 PM by John »
A malicious prosecution for a crime which never existed. An exposé of egregious malfeasance by public officials.
Indeed, the truth never changes with the passage of time.