Answer:
maturation
Step-by-step explanation:
Internal validity is simply how much the observed results represent the population and are not affected by errors or biases such as maturation which is threat to internal validity. Maturation is an internal validity threat that occurs when a participant's psychological and mental processes or physical changes internal to the participant interferes with evaluation results . This is seen in spontaneous remission where a person recovers from a disease which is rare to recover from and is therefore not representative of observation results