Hello everybody,

excuse me for my english. I am a french resident and i am doing a research about the evolution of several measures of an anatomical structure (diameter, length) in a small population (n=10, but less observation for several measures)
I need your help for understanding some biostatistical concepts and their interpretation.

A the beginning of my study, I oftenly used the ttest to compare this measures with mean +/- sd [min;max] avec p-value.
But recently, I "discovered" the xtmixed in stata. And for my measures the pvalue is not the same. The interpretation and the conclusion of my study could be very different so...

I asked me if it is better to use xtmixed or ttest for this. And how we can extract result of the xtmixed model.

My exemple:

TF_long= femoral tunnel length (A1 the first measure and A2 the second)
tunnel_type_femoral= type of the tunnel = 2 groups : epiphyseal (gr 1) and transphyseal (gr 2)

for ttest :
[ATTACH=CONFIG]temp_21134_1611820317847_900[/ATTACH]

for xtmixed i realize the test for the patient in group 1

[ATTACH=CONFIG]temp_21132_1611820188071_931[/ATTACH]


then in group 2
[ATTACH=CONFIG]temp_21133_1611820305552_590[/ATTACH]

my interpretation
fort ttest : there is no significant difference when we compare the first and second measures in all groups (p=0.0518), in group epiphyseal (p=0.1994) and in group transphyseal (p=0.0610)

but for xtmixed, there is an effect ... with p < 0.05
- when we compare in group epiphyseal : there is an augmentation by 0.83333 +/- 0.36 [0.13 ; 1.54] between the first and the second measure (p=0.021)
- when we compare in group transphyseal: there is an augmentation by 7.92 +/- 2.74 [2.54887 ; 13.3] (p=0.004)

Am I wrong of the interpretation of xtmixed model ?
Can I use this ?
If there other "stata expression" to combine the several group in one table ?

because i tried to use xi : xtmixed TF_long_A i.time_analyse i.tunnel_type_femoral || id_patient:, noretable

but the first categorical is omitted
(the 3th is a particular group that I don't want to analyse because of anatomical specificities)
[ATTACH=CONFIG]temp_21135_1611821326312_77[/ATTACH]

Thank you very very very much in advance for the help you will give me )