Thread: appropriate tests for randomly selected data in spss?

Hi. I am having troubles with what test should I use in comparing randomly selected data using spss?

Cr concentrations----------------Cr concentration absorption
15.00 mg/kg-------------------------------3.54
45.00 mg/kg-------------------------------4.27
75.00 mg/kg-------------------------------2.63
105.00 mg/kg------------------------------3.76
135.00 mg/kg------------------------------2.23
165.00 mg/kg------------------------------3.88
195.00 mg/kg------------------------------6.39
225.00 mg/kg------------------------------8.18
255.00 mg/kg------------------------------9.15
285.00 mg/kg------------------------------5.30
315.00 mg/kg------------------------------4.33

I want to know a test that is much similar to One-way ANOVA. I cant use ANOVA because of too few cases. I want to know if there is a significant difference among Cr concentration absorptions. Thanks!

Followup question:

Which test is better for post-hoc tests? LSD, Tukey, or Games-Howell test?

Okay, so you have 2 continuous variables.
The first is something you control, while the second is a result.
How about... a linear regression?

Hello.

Cr concentrations = is the treatment that we have done for each pot of soil. we injected 15 mg/kg, 45 mg/kg of chromium and so forth to each pot of soil. We grow plants in each soil to see if chromium has an effect on plants.
Cr concentration absorption = is the amount that our plants absorb from each pot of soil.

We made it as triplicates (15-A mg/g, 15-B mg/kg, 15-C mg/kg), but due to budget constraints, we randomly selected from each concentration (we picked 15-B mg/kg, 45-C mg/kg, 75-A mg/kg and so forth).
I want to know if there is a significant difference among Cr concentration absorptions. I've been using SPSS software and I chose one-sample t-test, but the problem is i do not know what test value to use. If I could just enter this randomly selected data as means, so that I can use ANOVA for significant differences.
Any help is greatly appreciated.

Followup question:

Which test is better for post-hoc tests? LSD, Tukey, or Games-Howell test?

And also, I want to know which concentrations have the significant differences. thanks!

Hi, I took your advice to use linear regression and this is what showed in the output in SPSS. Please take a look at the attach document.
Where can I find the value in the results that indicate a significant difference or non-significant difference?
Please help me, this is for my thesis.

You have a table "Coefficients".

From it you can see that you have:

Predicted Cr measurement = 2.847 + 0.012 * Concentration

In particular for the line related to Concentration, you have:

B: 0.012
Sig: 0.075

Since Sig > 0.05, the statistical conclusion is that you have insufficient proof to be able to say that the Cr measurement depends on the Concentration (in a linear regression).



Let's take a look at a graph of your data.



I can see that around 200-250 there seems to be some effect.
Below 180 and above 270 looks like random noise.

I guess that should match expectations.
There is a concentration that gives an optimal absorption.
Lower or higher concentrations will give less effect.
For something like that, a linear regression is the wrong test.

I guess you'd need for instance a non-linear regression test for that.

I've been playing around with your data a bit, thinking what kind of curve you might want to fit.

The best I could think of, was a kind of bell curve (like the normal distribution) augmented with some constant effect.

Here's a result.



This is a fit with a curve of the form:



The resulting numbers are:
B0 = 3.33
B1 = 5.84
B2 = 239.4
B3 = 49.5

This would mean a maximum effect at a concentration of 239.4 mg/kg.
And the effect would have a spread more or less equivalent to a standard deviation of 49.5 mg/kg.

Btw, this is a non-linear regression.

I just realized there is yet another test you can do: the t-test on correlation.






The t-distribution for this test has n-2 degrees of freedom.

In your case, you have t = 2,011 and the corresponding p-value is p = 0,038.
In a 2-sided t-test with , this is not significant, although it would be significant in a 1-sided t-test.

Hello again, I've been meaning to use t-test but I just don't know how to interpret data.

I just want to know if there is a significant difference among the concentrations.

H0: there is no significant difference among the concentrations
H1: there is a significant difference among the concentrations

BTW, you used the non-linear regression to know what concentration does the plant achieved the optimal absorption? or what concentration does the plant is comfortable with? It can be useful

And also it would be very much appreciated if you know which concentrations are alike or not similar.

For example:

Statistics analysis (insert test) shows that Concentrations 15 to 165, and 285 to 315 mg/kg do not have a significant difference with one another while they differ from the 195-255 mg/kg which also do not have a significant difference with one another.

Thank you!!!

Well, you could do a one-way ANOVA with the 3 categories (or some other category choice) you mentioned.
But I'd advise against it.
It does not look good if you use your sample to massage the data into a form just so you can find significance.

I think a graph like the one I showed with the bell-curve model would look good in a report.
However, I don't know how to properly support it with a t-test or any other test.

Sorry if this doesn't really help you.

The first is something you control, while the second is a result.