Complex sentence: O'Keefe hypothesizes that this is due to a plateau effect, as 75% of respondents in 1979 already reported locking their doors; the only personal security measure not mentioned in a television advertisement to see a significant increase was getting a dog.
Split into simpler sentences: O'Keefe hypothesizes that this is due to a plateau effect, as 75% of respondents in 1979 already reported locking their doors. The only personal security measure not mentioned in a television advertisement to see a significant increase was getting a dog.