Good morning all from a very gloomy Ireland where we have just reported our highest daily number of C-19 cases.

I have a quick question on standardising variables using panel data. I have two waves of data (pre and during-Covid), with variables for each wave suffixed with 1 and 2 e.g. jobsat1 and jobsat2. I am using a fe regression model to examine the impact of Covid-19 on different outcomes (mental health; job satisfaction etc). In order to be able to compare the effects I need to standardise the variables . To do that I started with the data in wide format and generated zscores using the following code.

egen float zjobsat1 = std(jobsat1), mean(0) std(1)
egen float zjobsat2 = std(jobsat2), mean(0) std(1)

I also used the code zscore jobsat1 /// zscore jobsat2 and it produced the same results

To conduct the fe regression, I reshaped my wide data into long format and applied xtset (id wave). This gave me a variable zjobsat.

My problem is that when I look at the difference in the zscores for my jobsat variable (and indeed all my other outcome variables) between waves 1 and 2 (i.e. zscore2-zscore1), the results are very weird I.e. the wrong sign and there is no significant changes at all when I run ttests. e.g. When I run a ttest on the raw scores (in red below), followed by a ttest on the zscores (in green below):

. ttest jobsat2==jobsat1

Paired t test
------------------------------------------------------------------------------
Variable | Obs Mean Std. Err. Std. Dev. [95% Conf. Interval]
---------+--------------------------------------------------------------------
jobsat2 | 618 2.425017 .0219672 .5460965 2.381878 2.468157
jobsat1 | 618 2.535626 .0225456 .5604743 2.49135 2.579901
---------+--------------------------------------------------------------------
diff | 618 -.1106083 .0163644 .4068135 -.1427451 -.0784716
------------------------------------------------------------------------------
mean(diff) = mean(jobsat2 - jobsat1) t = -6.7591
Ho: mean(diff) = 0 degrees of freedom = 617

Ha: mean(diff) < 0 Ha: mean(diff) != 0 Ha: mean(diff) > 0
Pr(T < t) = 0.0000 Pr(|T| > |t|) = 0.0000 Pr(T > t) = 1.0000

. ttest zjobsat2==zjobsat1

Paired t test
------------------------------------------------------------------------------
Variable | Obs Mean Std. Err. Std. Dev. [95% Conf. Interval]
---------+--------------------------------------------------------------------
zjobsat2 | 618 3.26e-10 .0402259 1 -.0789963 .0789963
zjobsat1 | 618 -.0005655 .040252 1.000648 -.0796129 .078482
---------+--------------------------------------------------------------------
diff | 618 .0005655 .0295705 .7351097 -.0575054 .0586364
------------------------------------------------------------------------------
mean(diff) = mean(zjobsat2 - zjobsat1) t = 0.0191
Ho: mean(diff) = 0 degrees of freedom = 617

Ha: mean(diff) < 0 Ha: mean(diff) != 0 Ha: mean(diff) > 0
Pr(T < t) = 0.5076 Pr(|T| > |t|) = 0.9847 Pr(T > t) = 0.4924

You will see that the t-score changes sign and magnitude and the difference between the figure for the two waves is no longer significant. I am not very familiar with z-scores in general and am at a loss as to why this is happening. Is it something to do with me giving separate zscores to jobsat1 and jobsat2? Do I need to standardise across both waves simultaneously and if so, how do I do this using Stata code?

I assume that I am doing something probably fairly basic wrong so any advice on how to fix this problem would be greatly appreciated.

Best wishes
Diane