# Statistical probability to help with a crime

1. Jul 5, 2010

### mysh

Hi There,
My maths used to be a lot better than it is these days, i gave up being a computer geek to set up my own business about 7 years ago and i can't work out the following as probability due to this and i'm rather stressed/angry, my attempts with excel are getting me nowhere

tonight people unknown attempted to break into my business premises and i want to calculate the probability of it being an inside job. my gut feeling is it is but it would be good to have the numbers to back it up.

here are the variables

i have had premises on this estate since october 2005 (1723 days) and this is the first time there has been any attempt to break in

there are the main door (which locks and unlocks with a yale type key) and a concertina gate (which automatically clicks locked but needs a key to unlock) in front of this. Tonight for the second time one of the lads who has keys has left another of the lads (who doesn't) in until he has finished working so he can leave only locking the outer door. so two people know that the inner more secure door is unlocked.

the last worker left the premises at 18:50 and by 23:50 that evening the outer lock has been forced with a tool for the job, it's quite a secure lock. no other locks on the other 51 industrial units have been targeted and show no sign of damage

luckily i had to nip back to the unit earlier in the evening and had locked the inner door. the inner door handle is reachable when the gate is shut so it seems that the persons who have done this expected the inner door to be open

there have been 3 other occasions in this time when i have forgetten to lock the inner door but no-one else would have known about these instances but they do affect the outcome of probability

i would be greatly appreciative if someone could crunch the numbers for me and let me know the probability of it being

• sheer coincedence
• inside job or at least outside influence of the knowledge the unit wasn't as secure as usual (1:1 is my guess at this one)

thanks again

Jerry

2. Jul 6, 2010

3. Jul 6, 2010

### The Chaz

Maybe you should invite the employees to a poker game...?!?!

4. Jul 6, 2010

### mysh

cheers for the comments guys, i imagine the probability of someone just happening to attack my premises in this timely manner is slightly longer than 44% TBH

I find poker to be like roulette, everyone else seems to leave the table better off than me.

I have removed the offending member of staff from our payroll so we will see if we have any other attempts.

I'm glad i found this site though so i'm off to the astronomy section now

cheers
Jerry

5. Jul 7, 2010

### CRGreathouse

Good luck to you.

6. Jul 10, 2010

### winterfors

I would say the chance of the burglar thinking that that the inner door was unlocked seems quite high in this case, significantly higher than the "less than-1% chance" that is would be unlocked at random.

Whether is was inside knowledge or something else (e.g. watching the last person leave without locking the inner door) that made him think that is difficult to judge from the information above.