Hypothesis testing and how to remember the difference between a Type I and Type II error

If you are doing research in the social sciences, or even if you’re not but like a bit of geek-based Indie music then this post is for you. If neither of those apply then I will forgive you for not reading the rest of this post (though it is worth checking out the very bad but catchy song at the end!), but in an effort to maintain my love of blogging along with my Masters Degree I am going to try and post a bit more psychology related stuff. Unfortunately for all my current module is statistics and research methods, so that is today’s topic.

A quick summary for those who don’t know, when we perform research in psychology we use a method called hypothesis testing, where we set a null and alternate hypotheses. The alternative hypothesis is always our prediction that there will be an effect of what we are measuring. The null hypothesis is always that there is no effect, and basically we are testing the assumption that there is no effect or difference in what we are measuring. Let’s give an example; I think doing crochet is more relaxing than watching football. I could design an experiment where I had a group of people watch football for an hour and a group of people do crochet for an hour, and then I could give them a test that measures relaxation and see if there is a difference. Now, this is an experiment at it’s simplest level, and there are many potential problems with it, feel free to comment on what they are, think of it as a crash course in Research Methods! Anyway, for that experiment our null hypothesis is “there is no difference in the level of relaxation attained by watching football or by doing crochet”, and the alternative would be “doing crochet makes you more relaxed than watching football”.

The reason we have the null is that we can never prove anything with statistics, we can only reject the null which supports our alternative hypothesis. We calculate the probability of our observation occurring if the null hypothesis is true, that is, what are the chances of getting this effect if there is really no difference in the things we are measure. This method is actually often criticised, as in real life if we take two measures of anything there is almost never no difference between them. We still do it this way though, but it highlights the importance in understanding the mechanisms underlying statistics and not just blindly accepting numbers that a computer spits out.

So, in psychology we determine the probability of obtaining a result at least as big as the one we obtained if the null hypothesis is true and use that to decide if we have a significant effect of not. We use the figure p=.05 as our cut off. Basically, if the statistics say there is a less than 5% chance of getting our observation if there is no effect we are comfortable enough to say “yep, guys, we have a significant effect here”. So if in our experiment above our p value for the differences between our groups (as calculated by a delightful programme called SPSS) is .02 it is basically saying “Look, if there really was no difference between the two groups you’ve got 2% chance of getting this result; that’s pretty low so probably there is a significant difference – reject the null hypothesis, reject I say!”. Anything up to 5% and we are comfortable that the difference is significant (yes, that figure is pretty arbitrary, and yes, there are many things wrong with it. What is the p value was 0.056? Well, it would be classed as non-significant for most academic journals).

However, even with our 5% p value there is still obviously 5% chance we could get our observation that makes it look like crochet is more relaxing than football, when really it isn’t. Maybe we just managed to find for our study the few people in the world who find crochet really relaxing, but the majority don’t. This idea that we might falsely reject the null hypothesis is called a Type I error. Or course, we may have got results that exceed our hallowed %5 probability thus causing us to accept the null hypothesis as true, there is no difference between the relaxing properties of crochet and football, when in fact there is a difference, we just didn’t pick it up in our study (maybe we didn’t look at enough people, or we didn’t make them do enough crochet…). This failure to reject the null hypothesis is known as a Type II error.

Now, why are these things important? Well mainly because I have an exam on such notions in a couple of weeks…but really because this is the basis of all social science research. Why am I writing a blog post on this? Well, those of you who have read and understood this probably already know it anyway from a basic research methods course. If you didn’t already know it then you can’t possibly have any reason to need to know it so I am impressed you persevered this far!m(Or maybe you are a student who does need to know but hasn’t understood it from your course, nor discovered a decent stats book like Discovering Statistics Using SPSS -seriously, this is a cracking stats book). Anyway, typing this all up has been great revision for me, which is mainly why I did it, and, well it’s my blog and I can write what I like! But what I really wanted to share was an “oh so bad it’s really good” song which someone, in the crusade to remember which way round Type I and Type II errors are, has written. Trying to remember which is why is a real pain, even after all these years (I first learned this stuff at A level) and is clearly an issue that plagues students the world over. For those who can’t make out the lyrics they are as follows:

If the null is zero
And it’s really zero
But you think it’s bull
And reject the null
Type I

If the null is zero
And it’s really not
And you accept the null
That’s off the spot
Type II

Isn’t it ace? I’m going to be singing this in my exam next month!

Do you have flower power?

Crochet flower
Crochet flower from this month’s Simply Crochet Magazine

In July my lovely friend and partner in crime Georgia is returning to her native land. I am desperately sad that she is going. In the 2.5 years since we randomly latched on to each other at a local sling meet she has introduced me to the delights of mac’n’cheese and white cake, and I have taught her to crochet and introduced her to yarn bombing.

In honour of her departure and to mark our friendship we are of course planning a yarn bomb. Flowers are the theme. We are going to coat a nearby deserving location with as many flowers as we can hook. And we are hoping for input from others too. Whether you are local to the Gloucestershire area and fancy helping us tie hundreds of flowers to railings or just want to contribute your own knit or crochet flora and fauna we’d love to hear from you. Big, small, any colour, we want your flowers! If you do want to be part of this yarn bomb (estimated date 3rd July) then email me at dillytanteblog@gmail.com or @dillytante on Twitter.

There are loads of flower patterns online, and I posted about some here. Many such as this one are super quick to make. Or the above flower was one I made this morning from my new Simply Crochet Magazine.

I haven’t given much notice, in my usual disorganised way. In my defence I have been preparing for my college residential which was last week, and my stats exam (97%, thank you very much!). Anyway, we’ll be grateful to have anyone join in and will keep you updated.

A middle class a-fair

Bank holidays are usually bittersweet for me; a long weekend in which you are supposed to chill, but if you are like me the pressure to Do Something Worthy gnaws away at any time not spent in pursuit of merriment or at least spring cleaning.

We were supposed to be camping this week, but our regular inspection of the BBC 5 day forecast has forced us to concede defeat. I make no bones about being a fair weather camper. Roughing it in a field with portaloos and two young kids (4 if you count the family we were supposed to be going with) is just about bearable, verging on fun when you have glorious weather and copious amounts of alcohol. In 22 MPH winds, Baltic temperatures and rain it is about as appealing as attending the UKIP party conference.

As a salve to the wounds of disappointment we decided to camp out in the garden night before last. All the benefits of camping, sleeping under canvas (well, some kind of nylon material anyway), fresh air, without the hassle of packing up the car and using chemical toilets. But er, we still had to put up the tent, which fit with inches to spare…

20130528-105343.jpgIt was good fun, though by 10.30pm when the kids were still awake it was hard to resist the temptation to pack up and chuck the kids back in their rooms. But we stuck it out, you know, coz we’re hardcore.

The bank holiday Monday was spent at the achingly middle class Suffolk Street Fair. It’s events like this that make me oscillate between contempt and intense life envy. The Suffolks are a slightly Bohemian, vair middle class area of an already quite middle class town. The fair consists of stalls from lots of local businesses; a mixture of art and craft, poncey food, and car boot sale tat with “vintage” prices.

We met some of our friends at the fair, with possibly cuter kids than ours – certainly more well dressed than our dress-refusnik girls!

Kids at the street fair
Consulting map apparently. Actually a leaflet on organic locally produced sausages.

Every year DH and I wander round and wish we could casually pick up a locally designed art print or a £50 distressed wire magazine rack, while at the same time scoffing at the “saw you coming” street sellers. See that is the fundamental (and really the only difference between us and the rest of these hipsters, most of them are richer than us. Our part time public sector salaries and lack of period property are the only things that stop us from becoming Guardianista cliches, and means that we get to play the boy who points out that the Emperor is wearing no clothes, and that the shabby chic piece of driftwood hanging from a bit of twine is, well, just a piece of driftwood. Don’t get me wrong, we want to buy the driftwood, we just can’t afford it.

Man selling beer
£4 for a can of larger? Saw you coming…

There is always good food at the fair. It was difficult to chose between the five vegetable tagine, the falafel and fresh pita bread, or Thai noodle. The spicy noodles won out, as they do every year. I wish I knew what spices they used as they were delicious, even if they did cost £4.50.

The sun was shining, and the jazz band was playing. One of the joys of having young kids is never being without a dance partner. It was just Betty and I throwing some moves, but I didn’t care.

Betty and I dancing in the street
Dancing in the street
Punch and Judy
Beating with a stick – that’s the way to do it!


A Punch and Judy show kept the kids bizarrely enthralled, in the way only the iPad usually does. Well, what child can resist watching a scary wooden puppet, with the bulbous nose of the inebrient, whack a dog with a wooden stick and get whacked in return. The children cackled with laughter at 50 Shades of Candy Stripes while cringing lentil weaver parents shifted uncomfortably from one foot to the other, clutching their recently purchased organic olive oil and wondering how to explain away the gratuitous violence to their kids later. We didn’t stay to find out the fate of Judy.

We dragged the kids away from the bunting clad street, stopping at the fab charity book shop on the Bath Road. Betty chose six Enid Blyton books (she is undoubtedly her mother’s daughter) – ‘vintage’ as per the order of the day, although as DH found out later when he read them, vintage books tend to come with vintage attitudes to race, women, foreigners, poor people, and basically anyone who doesn’t own an island. I. however, found some light bedtime reading, which I can guarantee contains no black people called Sooty…

Statistics without maths book
It says without maths – that’s go to be good right?

All in all a pleasant day. And while we traipsed along the street fair fantasising about owning one of the Regency townhouses, with wooden shutters and shabby chic decor, it wasn’t unhappily that we returned to our rented little new build terrace, with laminate floors and magnolia walls, just with realism, and the feeling of familiarity. We can’t pull off vintage anyway.

 

If you can cope with bringing up kids, anything else is easy in comparison

I’ve been working at my MSc Occupational Psychology for nearly 6 months now, and it is hard, but actually easier than I thought it was going to be. On my course I am the only student to have children, and I have received nice comments like “when I am struggling to fit it all in I think if Dilly can do it with 2 kids then I can do it” but in actual fact I really believe that being a mother of young children has helped me manage this course, for the following reasons:

I already have no life
Some of my student friends are struggling to fit studying in with all they things they usually do in their free time, and the endless weddings and hen nights that take up the weekends of those of a certain age. Maybe it’s not even age, I don’t think I’m the oldest, but I happen to be in a situation where most of my friends are already married. I think some of the students are finding it a shock that they have to sacrifice nights out and weekends away for sitting down and studying. Well, I have kids so I’ve already sacrificed those things. I have already been through the pain of feeling my freedom restricted. Iris isn’t really reliable enough for a babysitter, and even if she was we couldn’t afford it anyway. So for me, most of my evenings are a toss up between studying or watching the West Wing and crocheting on the sofa. Some things have had to go. You can see that my blog is a bit neglected, and I’m having crochet withdrawal, but I have experienced the feelings of sacrifice already and I know it isn’t forever.

Every second counts
What the hell did I do with my time before I had kids? Obviously I worked full time (but I’m not far off that now), but it’s not like I was writing War and Peace. I wasn’t even reading it. We had dinner parties with friends, did a bit of jogging, but again I still manage that now. All those child-free hours, I could have spent doing something useful but with the naivety of youth I just frittered them away. Now every hour is accounted for, and if I am lucky enough to have “free time” every minute is squeezed dry. Because of this when I sit down to do my work I’m very conscious of time. I know how many hours I need to spend on my studying, and how many hours I have available in the week, and there is little slack. If one of the kids is sick for a couple of days that writes off a few evenings of work that I can’t afford to lose so I know I have to keep on top of things.

Less pressure to be top
I did really well in my first two degrees, a First and a Distinction. Anything less in this one is going to feel like a step back. Academia is my thing. I nearly cried when I got 55 in my first assignment. But what with combining a nearly full time job, two kids and other activities with this degree, everyone is just going to be impressed if I pass. I’m nearly coming around to that view myself. Nearly.

It’s not the hardest thing I have ever done
I survived 10 months and more without a full night’s sleep. I have breastfed while suffering from an excruciating migraine, delatching the baby to go and vomit, then returning to resume a prone position while a tiny baby sucked the life force out of me. I have driven through the night to get a baby suffering from chicken pox to stay asleep. I have cared for a sick husband and toddler a week after giving birth. I have given birth. Twice. With no drugs. I have gone to work leaving my children in the care of virtual strangers for the first time. I have raised two charming and clever children. In terms of the hardest things I have done, a part time degree is not even up there.

Everyone thinks I am doing an amazing job
There is nothing quite as motivating as praise from other people, and lots of people have expressed their admiration at what I am doing. My mum and dad have both said how proud they are, as has my husband. And my step-mum went so far as to give me a significant chunk of money towards my course, because she felt I really deserved it. When really, as I have just explained, in some ways it is easier for me than everyone else, you know, what with having no life and all. Blown that myth now haven’t I?

And on top of all that it helps that I love psychology, really want a new job, and am fortunate enough to be fairly bright. My reason for writing this post is really to inspire other people out there to push their boundaries, especially other parents. I worried for ages about whether I could cope with doing this course. Yes I’m a bit grumpy sometimes, I feel like I have no time to decompress, but it will all be worth it in the end. And as with most things in life, it hasn’t been as hard as I feared. So if you are thinking of taking something on, and are wondering how you would cope when you have children, my answer is this – having kids: probably the hardest thing you will ever do. Whether you are thinking of doing a degree, starting a business, writing a book, it’ll be easy in comparison. And by virtue of the the skills you will have picked up just from having kids, you will be even better equipped for whatever you take on.