Nigel Holt and Rob Lewis - Crown House Publishing.

DATA ANALYSIS AND REPORTING INVESTIGATIONS | 23Probability and significanceWe’ve already let you in on a few secrets aboutstatistics. Here’s another. All they are for is forshowing that your result didn’t occur by chance.That’s pretty much it. Don’t let anyone fool youinto thinking they are any more complicated thanthat. When we do some statistics all they give usafter we’ve finished adding, multiplying, dividingand things is a single important number, which werefer to as ‘p’. It stands for ‘probability’.The p value gives us an idea of how likely it isthat our results happened by chance or not. Thep value can be anywhere between 0 and 1. A p of1 means that something is definitely, absolutelygoing to happen. For instance, there is a probabilityof 1, an absolute certainty, that in the UKChristmas Day will fall on December the 25th. Ap of 0 means that something is never ever goingto happen. For instance, there is a probability of 0that this book will turn into a badger.The nearer to 1 your value of p is, the more likelyit is that the results happened by chance. Thesmaller the value of p, the more likely it is thatwe can confidently accept our hypothesis, andreject our null hypothesis. A p of 1 means that weare 100% certain that something will happen bychance. A p of 0.9 means that we are 90% certainthat something will happen by chance. A p of 0.8means we are 80% certain that something willhappen by chance and so on. The smaller the pthe less likely it is that something will happen bychance. Remember:Small p – GoodBig p – BadIf we can reject our null hypothesis we can say thatour results were ‘significant’. By this we mean thatour findings did not occur by chance. Statistics letus measure how ‘significant’ our results are. Statisticaltests are used to analyse the data gatheredin research to tell us the p value (we will describethese tests in more detail shortly). The smallerthe p value, the more significant the result. Forexample, the statistical analysis might tell us thatwe have a p value of 0.02. This is the significancelevel. When we report ‘p’ in research papers andbooks it is more often than not written like this:p