Be aware of the proper use of single and double quotation marks when copying Stata or other codes from this blog (but also when copying codes from other websites). Often opening or closing quotation marks are wrongly formatted.
Double quotation marks in Stata (“) are written using the button (shifted) left of the enter-button (both opening and closing quotation marks
Single quotation marks in Stata (`and ‘): opening single quotation marks are written using the button in the top-left corner next to the esc-button. Closing single quotation marks are written using the button left of the enter button on the keyboard (the same as for double quotation marks, but not shifted).
For detecting mistakes in setting quotation marks it also helps to install Stata-syntax highlighting which is available for many text-editors, such as WinEdt. For installation help, click here: http://www.winedt.org/Config/modes/Stata.php