Mathematics: Post your doubts here!

[Dug]

Anyone? PhyZacDug

Class Intervals | X
1 - 100 | 50.5
101 - 150 | 125.5
151 - 200 | 175.5
...

 

[Scafalon40]

I had same doubt, but apparently the reason is, it is less than or equal to the specific value. When it is only less than then only 50 not 50.5
i guess! not sure.

Class Intervals | X
1 - 100 | 50.5
101 - 150 | 125.5
151 - 200 | 175.5
...

Interesting. Dug's method make sense. Thanks for sharing, Dug! Any thoughts, PhyZac?

 

[Scafalon40]

http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Mathematics (9709)/9709_s11_qp_63.pdf
ms http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Mathematics (9709)/9709_s11_ms_63.pdf
Have a look at Q3 part iii. Could you tell me the class boundries for this one Dug
I think its something like 19.5-34.5 etc. No?

 

[daredevil]

http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Mathematics (9709)/9709_s11_qp_63.pdf
ms http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Mathematics (9709)/9709_s11_ms_63.pdf
Have a look at Q3 part iii. Could you tell me the class boundries for this one Dug

i thikn it will be like for the first one: 0=< x < 10

include the first value and not the last one.... there can be no decimals i think bcz there are marks

 

[Dug]

http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Mathematics (9709)/9709_s11_qp_63.pdf
ms http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Mathematics (9709)/9709_s11_ms_63.pdf
Have a look at Q3 part iii. Could you tell me the class boundries for this one Dug
I think its something like 19.5-20.5 etc. No?

0 - 9
10 - 19
20 - 34

 

[Scafalon40]

0 - 9
10 - 19
20 - 34

Yeah exactly. So the boundries are something like:
-0.5-9.5
9.5-19.5
19.5-34.5
Correct or not?

 

[Scafalon40]

i thikn it will be like for the first one: 0=< x < 10

include the first value and not the last one.... there can be no decimals i think bcz there are marks

But that means the class width is 10, and there is no way you can get a midpoint of 4.5, as it says in the markscheme.

 

[Dug]

Yeah exactly. So the boundries are something like:
-0.5-9.5
9.5-19.5
19.5-34.5
Correct or not?

Class boundaries are not needed for calculating mean. We do that only in histograms and cmf curves!

 

[Scafalon40]

Class boundaries are not needed for calculating mean. We do that only in histograms and cmf curves!

You need the class boundaries to calculate the midpoint, don't you?

 

[Dug]

You need the class boundaries to calculate the midpoint, don't you?

Nope!
Class Interval | Mid-point | Frequency
0 - 9 | 4.5 | 25
10 - 19 | 14.5 | 43
20 - 34 | 27 | 91
35 - 49 | 42 | 75
50 - 69 | 59.5 | 26
70 - 99 | 84.5 | 40

Mean = (4.5)(25) + (14.5)(43) + (27)(91) + (42)(75) + (59.5)(26) + (84.5)(31) = 11270/300

 

[Scafalon40]

Nope!
Class Interval | Mid-point | Frequency
0 - 9 | 4.5 | 25
10 - 19 | 14.5 | 43
20 - 34 | 27 | 91
35 - 49 | 42 | 75
50 - 69 | 59.5 | 26
70 - 99 | 84.5 | 40

Mean = (4.5)(25) + (14.5)(43) + (27)(91) + (42)(75) + (59.5)(26) + (84.5)(31) = 11270/300

Dug I'm pretty sure you calculate the class width by using the class boundaries. Then using the class width you can calculate the midpoint! If not, could you provide me with some reference stating otherwise?

 

[Dug]

Dug I'm pretty sure you calculate the class width by using the class boundaries. Then using the class width you can calculate the midpoint! If not, could you provide me with some reference stating otherwise?

I don't have a reference but you could try calculating the mean using your method. The answer will come out wrong!

 

[PhyZac]

Interesting. Dug's method make sense. Thanks for sharing, Dug! Any thoughts, PhyZac?

Hmm, I will have do understand it first. But seems very sensible.

 

[daredevil]

Yeah exactly. So the boundries are something like:
-0.5-9.5
9.5-19.5
19.5-34.5
Correct or not?

but the marks cannot be in negative :O

 

[Scafalon40]

I don't have a reference but you could try calculating the mean using your method. The answer will come out wrong!

Actually it would be correct. You can try yourself
It's the midpoints that matter, Dug. So take the class 19.5-34.5. It's midpoint is 27. And so on. Do you have that stats book by Chambers and Cranshaw?

 

[Scafalon40]

but the marks cannot be in negative :O

Yeah I know, and that's what confuses me the most!

 

[PhyZac]

Yeah I know, and that's what confuses me the most!

For 0 it stays zero, keep it like a rule.!!

 

[Dug]

Actually it would be correct. You can try yourself
It's the midpoints that matter, Dug. So take the class 19.5-34.5. It's midpoint is 27. And so on. Do you have that stats book by Chambers and Cranshaw?

What about the mid-point of the first class 0.5 - 9.5?

 

[Scafalon40]

For 0 it stays zero, keep it like a rule.!!

Mate look: if you have two classes such as 10-19 and 20-29, then their class boundaries should be 9.5-19.5 and 19.5-29.5. That's what I was taught. I think you would concur.

 

[Scafalon40]

What about the mid-point of the first class 0.5 - 9.5?

No. I was thinking of something along the lines of -0.5-9.5. You need class boundaries! The negative confuses me, yes. The only possible reason I can think of is negative marking
On a serious note, I don't quite understand the negative part.