Unless you look at things like app reviews. Where users are much more likely to complain than to leave positive reviews. I think the same applies for other +1/-1 options. People that don't like things run around jumping and yelling how much they hate it. Most of the ones that have positive things to say don't unless you put it right in front of their face.
- A dissatisfied customer will tell 9-15 people about it. And approximately 13% of your dissatisfied customers will tell more than 20 people about their problem. Source: the White House Office of Consumer Affairs, Washington, DC.
- Happy customers who have their problems resolved will tell 4-6 people about their positive experience. Source: the White House Office of Consumer Affairs, Washington, DC.
