Hello everyone!

I am using the asreg command to calculate betas using one year of daily data, but I want the year to have at least 150 observations. (I am using CRSP daily data)
To that end I applied the following :

gen day = day(date)
bys PERMNO : asreg exc_ret MktRF, window( day 252 ) min(151)

It works, however, I get values even for the very first observations of my sample.
Does anyone have any idea why that happens?

Thank you for taking the time to read my question.

P.S: In case it is relevant, here is a preview of my data (before I added the MktRF and before I created the exc_ret) using dataex.

. dataex

----------------------- copy starting from the next line -----------------------
Code:
* Example generated by -dataex-. For more info, type help dataex
clear
input double PERMNO long date double(SHRCD EXCHCD) str8 CUSIP double(PRC RET SHROUT)
10001 12057 11 3 "36720410"    14.5    .0357142873108387 1080
10001 12058 11 3 "36720410"  -14.25 -.017241379246115685 1080
10001 12059 11 3 "36720410"  -14.25                    0 1080
10001 12060 11 3 "36720410"    14.5  .017543859779834747 1080
10001 12061 11 3 "36720410"    14.5                    0 1080
10001 12064 11 3 "36720410"    14.5                    0 1080
10001 12065 11 3 "36720410"  -14.25 -.017241379246115685 1080
10001 12066 11 3 "36720410"   13.75 -.035087719559669495 1080
10001 12067 11 3 "36720410"   13.75                    0 1080
10001 12068 11 3 "36720410"     -14    .0181818176060915 1080
10001 12071 11 3 "36720410"     -14                    0 1080
10001 12072 11 3 "36720410"   13.75  -.01785714365541935 1080
10001 12073 11 3 "36720410"   14.25     .036363635212183 1080
10001 12074 11 3 "36720410"   14.25                    0 1080
10001 12075 11 3 "36720410"   13.75 -.035087719559669495 1080
10001 12078 11 3 "36720410"     -14    .0181818176060915 1080
10001 12079 11 3 "36720410"     -14                    0 1080
10001 12080 11 3 "36720410"   13.75  -.01785714365541935 1080
10001 12081 11 3 "36720410"     -14    .0181818176060915 1080
10001 12082 11 3 "36720410"     -14                    0 1080
10001 12085 11 3 "36720410"   13.75  -.01785714365541935 1080
10001 12086 11 3 "36720410"   13.75                    0 1080
10001 12087 11 3 "36720410"     -14    .0181818176060915 1080
10001 12088 11 3 "36720410"     -14                    0 1080
10001 12089 11 3 "36720410"     -14                    0 1080
10001 12092 11 3 "36720410"   13.75  -.01785714365541935 1080
10001 12093 11 3 "36720410"   14.25     .036363635212183 1080
10001 12094 11 3 "36720410"   13.75 -.035087719559669495 1080
10001 12095 11 3 "36720410"     -14    .0181818176060915 1080
10001 12096 11 3 "36720410"   14.25   .01785714365541935 1080
10001 12100 11 3 "36720410"      14 -.017543859779834747 1080
10001 12101 11 3 "36720410"     -14                    0 1080
10001 12102 11 3 "36720410"   14.25   .01785714365541935 1080
10001 12103 11 3 "36720410"     -14 -.017543859779834747 1080
10001 12106 11 3 "36720410"     -14                    0 1080
10001 12107 11 3 "36720410"     -14                    0 1080
10001 12108 11 3 "36720410"   14.25   .01785714365541935 1080
10001 12109 11 3 "36720410"     -14 -.017543859779834747 1080
10001 12110 11 3 "36720410"   14.25   .01785714365541935 1080
10001 12113 11 3 "36720410"    14.5  .017543859779834747 1080
10001 12114 11 3 "36720410" -14.125   -.0258620698004961 1080
10001 12115 11 3 "36720410" -14.125                    0 1080
10001 12116 11 3 "36720410" -14.125                    0 1080
10001 12117 11 3 "36720410" -14.125                    0 1080
10001 12120 11 3 "36720410"   13.75 -.015398230403661728 1080
10001 12121 11 3 "36720410"   13.75                    0 1080
10001 12122 11 3 "36720410"   14.25     .036363635212183 1080
10001 12123 11 3 "36720410"      14 -.017543859779834747 1080
10001 12124 11 3 "36720410"   14.25   .01785714365541935 1080
10001 12127 11 3 "36720410"  14.125 -.008771929889917374 1080
10001 12128 11 3 "36720410"   14.25  .008849557489156723 1080
10001 12129 11 3 "36720410" -14.125 -.008771929889917374 1080
10001 12130 11 3 "36720410"   13.75  -.02654867246747017 1080
10001 12131 11 3 "36720410" -14.125  .027272727340459824 1080
10001 12134 11 3 "36720410"    14.5   .02654867246747017 1080
10001 12135 11 3 "36720410"   13.75   -.0517241396009922 1080
10001 12136 11 3 "36720410"   13.75                    0 1080
10001 12137 11 3 "36720410"   13.75                    0 1080
10001 12138 11 3 "36720410" -14.125  .027272727340459824 1080
10001 12141 11 3 "36720410"   13.75  -.02654867246747017 1080
10001 12142 11 3 "36720410" -14.125  .027272727340459824 1080
10001 12143 11 3 "36720410"   14.25  .008849557489156723 1075
10001 12144 11 3 "36720410" -14.125 -.008771929889917374 1075
10001 12145 11 3 "36720410" -14.125                    0 1075
10001 12148 11 3 "36720410"    14.5   .02654867246747017 1075
10001 12149 11 3 "36720410" -14.125   -.0258620698004961 1075
10001 12150 11 3 "36720410" -14.125                    0 1075
10001 12151 11 3 "36720410"   13.75  -.02654867246747017 1075
10001 12155 11 3 "36720410" -14.125  .027272727340459824 1075
10001 12156 11 3 "36720410"    14.5   .02654867246747017 1075
10001 12157 11 3 "36720410"    14.5                    0 1075
10001 12158 11 3 "36720410"    14.5                    0 1075
10001 12159 11 3 "36720410"   14.75  .017241379246115685 1075
10001 12162 11 3 "36720410"   14.75                    0 1075
10001 12163 11 3 "36720410"   14.75                    0 1075
10001 12164 11 3 "36720410"   14.75                    0 1075
10001 12165 11 3 "36720410"    14.5 -.016949152573943138 1075
10001 12166 11 3 "36720410"   14.75  .017241379246115685 1075
10001 12169 11 3 "36720410"   14.75                    0 1075
10001 12170 11 3 "36720410"   14.75                    0 1075
10001 12171 11 3 "36720410"  -15.25  .033898305147886276 1075
10001 12172 11 3 "36720410"   15.75  .032786883413791656 1075
10001 12173 11 3 "36720410"  -15.25   -.0317460335791111 1075
10001 12176 11 3 "36720410"  -15.25                    0 1075
10001 12177 11 3 "36720410"    15.5  .016393441706895828 1075
10001 12178 11 3 "36720410"   15.75  .016129031777381897 1075
10001 12179 11 3 "36720410"  -15.25   -.0317460335791111 1075
10001 12180 11 3 "36720410"   15.75  .032786883413791656 1075
10001 12183 11 3 "36720410"  -15.25   -.0317460335791111 1075
10001 12184 11 3 "36720410"  -15.25                    0 1075
10001 12185 11 3 "36720410"    15.5  .016393441706895828 1075
10001 12186 11 3 "36720410"    15.5                    0 1075
10001 12187 11 3 "36720410"   14.75  -.04838709533214569 1075
10001 12190 11 3 "36720410"      15  .016949152573943138 1075
10001 12191 11 3 "36720410"    15.5   .03333333507180214 1075
10001 12192 11 3 "36720410"   15.25 -.016129031777381897 1075
10001 12193 11 3 "36720410"    15.5  .016393441706895828 1075
10001 12194 11 3 "36720410"    15.5                    0 1075
10001 12197 11 3 "36720410" -15.125 -.024193547666072845 1075
10001 12198 11 3 "36720410" -15.125                    0 1075
end
format %td date