IA Body Mass Index

[pomoni]

I want to ask if it would be okay to just take one of the sources found here on the website to take BMI statistics of women? They would't mind/know in the IBO, right? Because I really can't find anything on the net.

[llamajuice]

how do you create a function that models the behaviour of the graph manually?

[TNSGF]

You can google it to get more details for the actual solving part. I love PurpleMath. =)

I am doing the same thing but was unable to find any solving methods on the internet except a yahoo answers link.

Right now I have 5 equations, as you said. Do I just solve each one with the other to find the individual values of A-E?

And, will that not take for ever? Would I be expected to write that all out in my investigation?

[llamajuice]

when it says to use your model to estimate the BMI of a 30 yr old woman in the US, is it looking for the model we found ourselves in part 2 or the model we found with technology in part 4?

[TNSGF]

i used the model form part 2.

[ibibib1022]

It's polynomial of 3rd degree or higher. Should you choose this option, I've posted the instructions on how to obtain polynomial of any degree (up to maximum number of points in your set of data, to be precise) here. You will find data for median BMI of females here. Another option is to mess around with the sine or cosine function. I can also assist you with this one if you like, but I'd go for polynomials if I were you.

Hi... It's been a while since your post but I would really benefit from the link you attached if it worked.. Is there a way you could send it to me? Thank you

Hi....Soo I'm doing the BMI math portfolio.. which is due in 5 days and I'm kind of confused..

What does it mean by "Define all variables used an state any parameters clearly"? What are parameters?

And I chose to try modelling with a cubic function... But I don't remember how to find the equation using the points...

Some help would be greatly appreciated.. Thanks guys

Edited June 3, 2010 by Aboo

[Daedalus]

It's polynomial of 3rd degree or higher. Should you choose this option, I've posted the instructions on how to obtain polynomial of any degree (up to maximum number of points in your set of data, to be precise) here. You will find data for median BMI of females here. Another option is to mess around with the sine or cosine function. I can also assist you with this one if you like, but I'd go for polynomials if I were you.

Hi... It's been a while since your post but I would really benefit from the link you attached if it worked.. Is there a way you could send it to me? Thank you

Hi....Soo I'm doing the BMI math portfolio.. which is due in 5 days and I'm kind of confused..

What does it mean by "Define all variables used an state any parameters clearly"? What are parameters?

And I chose to try modelling with a cubic function... But I don't remember how to find the equation using the points...

Some help would be greatly appreciated.. Thanks guys

I did this portfolio and got full marks with a 5 page portfolio. I had three models. If you model it with a periodic function you will get a model that fits the data very closely but doesn't make sense when extrapolated, with a linear you will get the reverse. The fundamental problem with this is you can't predict peoples' BMIs when they are adults based on their childhood BMIs, because they're growing and their bodies aren't in proportion and what not. You just have to show the conscious ability to link mathematical models with what they really *mean* in terms of predictions and stuff.

I don't know exactly how to model with cubic functions (in fact I don't know how to do it at all) but modelling a sine function in the form AsinB(X-C)+D is pretty straightforward. I don't think you have to make it too complicated, better to make a bad model and explain why its so bad.

[sweetnsimple786]

Aron, that makes so much sense. I didn't explain it like that, but I did alright, since the teachers don't all grade the same. My friend did a sine wave, and my teacher took off major points, even though she explained that the sine function wasn't reliable. I did a cubic function, I think.

You just need to say y=ax³ + bx² + cx + d

and solve for a, b, c, and d by plugging in data for x and y =)

[tahsgirl]

Hi all,

I agree that this portfolio is probably the least evil of the type 2's, however my class is having a small dilema. We haven't covered matricis yet and my teacher says that solving simultaneous equations with 4 variables is not a part of the SL syllabus, so we have no way of determining the formula manually. The portfolio itself does not directly say to find the first function manually but I can see the benefit of doing that...however, I have no idea how too. If somebody could post either a link that actually works or clear instructions as to how to solve simultaneous equations with 4 variables that would be very helpful.

Thanks!

[Daedalus]

think about it isn't it just the same as solving a simultaneous equation with 2 or 3 variables?

suppose you have the general formula

y = ax³ + bx² + cx + d

and the specific points (0,0) and (8,10) and (16,25) and (-8,10).

you substitute 0 in for x and 0 for y and you get that d = 0, so the equation becomes

y = ax³ + bx² + cx

then you take (8,10) and plug it in, so 10 = 1000a + 100b + 10c

and you take (16,25) and plug it in, so 25 = 4096a + 256b + 16c

and you take (-8,10) and plug it in, so 10 = -512a + 64b - 8c

now this is just an example with random points from the top of my head, but you would start eliminating terms. for example say you double the third equation, you get:

20 = -1024a + 128b - 16c

this is useful because if you add this to the second equation, the c terms cancel out, leaving

45 = 3072a + 320b~

now to eliminate the b its a little more complicated, because you need another equation that doesn't have c in it (otherwise you have an equation with no b term but a c term...) so you have to use a different combination of equations, i.e. 1 and 3, to do this. if you multiply equation 1 by 8 and equation 3 by 10 and add them together, you get:

80 + 100 = 8000a - 5120a + 800b - 640b + 80c - 80c

which simplifies to

180 = 2880a + 160b

and now you have that equation with the other one, 45 = 3072a + 320b, so you double it (360 = 5760a + 320b) and find the difference, giving

360 - 45 = 5760a - 3072a + 320b - 320b

which simplifies to

315 = 2688a

now you get a = 315/2688 and you can plug that into the a and b equations to get c and then you have a and b so you can figure out c.

So the general method for solving a polynomial equation with multiple variables is to have several points which you can plug in, and substitute them into equations. then you use different equations to cancel terms out until you have one term in an equation, and when you solve for it, you can work backwards to find the other ones. it's not that complicated but it can be time-consuming i guess... also i don't know if it will actually work. but mathematically this way does give you a value for all the constants...

[laryxle]

I got a function for part 2, I chose a cubic function and used simultaneous equations to manually find the equation, however when I compare my model with the curve the data makes the only parts where they intersect is the four points I used in the simultaneous equations, and the two graphs really don't look alike at all. Is it better to just find a new function to model the data or is it ok if you keep with one that is obviously not correct? When, in part three, it says refine your model if necessary, does that mean say changing it to a quartic or sinusoidal function or just making really small changes?

EDIT: Actually nvm, I redid my calculations and got a really nice curve.

Edited June 12, 2010 by laryxle

[fervaldez]

Does anyone have any idea about what should be included in the IA front page? I'm not really sure about the format and all that stuff

[kingdomx]

Just finished this portfolio. Surprisingly, I found it easier than the previous, logarithm type I portfolio, as there was a lot less steps involved with this BMI portfolio. I started off using the sine function, because it is obviously better than the cubic function, as after an increase in age, the BMI would become negative (keeps going down). I then used the quartic for the technology function, and this equation was easy to find, as I just used the 'best fit' option in autograph. Obviously the quartic model didn't fit the BMI data of another country, as different countries = different cultures = different lifestyles = different BMI values. Limitations were pretty straight forward. Age, BMI, (weight and height) must all be larger than 0. There are basically unlimited limitations, and they are all based on logic. These limitations all affect the preciseness of the final model found.

[soccer]

i am doing this now, as a practice. and wouldnt the limitation be x>0? because no one can have a body mass index of zero. theyd be non-existent

[vinay427]

I would also use a polynomial function of >3 degree for the third step.