I bought the ovaluation (LH) Test Paper from Amazon.com and I think it's similar to David Paper but I think mine is more accurate. I don't like things made in China so I bought those made in Canada. Besides, I also bought Clear Plan ovulation test tube from Mannings' which is useful 'cos I used it for 2 months.
This month, when I used LH paper from Canada and clear plan together. On the same day, they both had positive symbols, so i think they're both useful and showed that i was ovulating.