Testing if the mean of a variable has changed significantly over time

[Will Hornby]

i am doing an assignment on bank mergers and want to see if there is a significant difference in the the mean of return on average assets 1 year before the merger announcement and 3 years after. I am using a panel dataset of 16 banks, have used xtset = company to set out my data as panel.
I am unsure as to what is the best approach to this; i thought a ttest would be at first, but after further research developed the following code and output with xtreg;

. gen byte era = 0 if time == 0

. replace era = 1 if time == 16
(15 real changes made)

. xtreg returnonavgassetsroaa i.era, fe

Fixed-effects (within) regression Number of obs = 31
Group variable: companyid Number of groups = 16

R-sq: Obs per group:
within = 0.0428 min = 1
between = 0.1344 avg = 1.9
overall = 0.0323 max = 2

F(1,14) = 0.63
corr(u_i, Xb) = 0.0418 Prob > F = 0.4418

------------------------------------------------------------------------------
returnonav~a | Coef. Std. Err. t P>|t| [95% Conf. Interval]
-------------+----------------------------------------------------------------
1.era | -.1293333 .1633765 -0.79 0.442 -.479741 .2210744
_cons | .9167742 .1127258 8.13 0.000 .6750014 1.158547
-------------+----------------------------------------------------------------
sigma_u | .30860528
sigma_e | .4474249
rho | .32237222 (fraction of variance due to u_i)
------------------------------------------------------------------------------
F test that all u_i=0: F(15, 14) = 0.90 Prob > F = 0.5785

can anyone please let me know if this is a correct way to assess this, and if possible help me with interpretting the tables. i have some idea but have limited experience in this field.

any help much appreciated.

[Carlo Lazzaro]

Will:
welcome to this forum.
See the policy of this forum on (graded) assignments at: https://www.statalist.org/forums/help#adviceextras #4.
I would recommend you to re-read -xtset- and -xtreg- entries in Stata .pdf manual.