The ratio of bullish responses (buy signals or messages) to bearish responses (sell signals or messages). Responses are also known as newsletters.

It can also be calculated as the number of buy messages minus the number of sell messages.

Sentiment index = no. of buy messages – no. of sell messages

In this sense, it measures the net bullish sentiment.

From another angel, it can be defined as the number of sell messages minus the number of buy messages:

Sentiment index = no. of sell messages – no. of buy messages

In this specific meaning, the sentiment index is a measure of the net bearish sentiment.