Looking for Pinyin standards
Note that sometimes the transcription reflects the actual reading rather than the "single-character-pinyin".

For example, 你好 is sometimes transcribed as ní hăo, even if the "actual" transcription per-character would actually be nǐ hăo. This happens because when two third tones are next to each other, the first one becomes a second tone (when reading/pronouncing the characters, not when writing). 

In other cases there is a totall loss of the tone, such as in 窗户 (window) transcribed — and read — as chuāng hu1, rather than chuāng hù.



1: The source is the CEDICT dictionary.