Fig. 1. Measures of confidence.

Confidence measures can be divided into general measures of confidence level and precision measures of confidence estimation. To assess someone’s general level of confidence, confidence level or calibration can be analyzed. Calibration (or confidence bias) is usually calculated as the difference between mean task performance and confidence. This results in overconfidence when confidence levels are higher than performance levels, and underconfidence vice versa. To assess someone’s precision of confidence estimation, confidence discrimination, metacognitive sensitivity or metacognitive efficiency can be analyzed. Confidence discrimination refers to the difference in confidence levels between correct and incorrect choices. The larger this difference, the higher the discriminatory accuracy of confidence, signaling an increased ability to recognize accurate from inaccurate performance by using one’s metacognitive report. Confidence discrimination is sometimes referred to as ‘the confidence gap’. Confidence bias and discrimination are two independent aspects of metacognition: an individual might be underconfident, but still be highly sensitive to discriminate between accurate and inaccurate performance with their confidence. Similar to discrimination, metacognitive sensitivity, also referred to as parameter meta-d’, aims to measure the ability of a metacognitive observer to discriminate between correct and incorrect trials with their confidence judgments. Yet, it uses a more sophisticated calculation that is bias free, and controls for performance confounds. On the other hand, metacognitive efficiency, referred to as meta-d’/d’, indicates how well perceptual information (d’) is used to form a metacognitive report (meta-d’). When meta-d’/d’, or the M-ratio, equals 1 (i.e. indicated by the line in the graph), this signals a metacognitively ideal observer that uses all perceptual information captured in d’ for the formation of a metacognitive report. When meta-d’/d’ < 1, not all information was used to form a metacognitive report, corresponding to lower metacognitive efficiency. When meta-d’/d’ > 1, the observer retrieved additional information to form a metacognitive report, corresponding to higher metacognitive efficiency