identified, and if the exposed to non-exposed ratio were increased (ex., four non-exposed participants for every exposed participant), it is doubtful if the statistical power would be sufficient to enable statistical conclusions to be drawn for any outcome except possibly thyroid nodules or thyroid deficiency. 5.4.9 Possible Selection Bias e Self-selection bias is possible due to voluntary participation in the medical surveillance program e Some persons moved to the larger populated areas (exposed and non-exposed) and could not be located. To the extent that these persons differ from those not relocating, (i.e., are healthier, better educated, etc) selection bias may be introduced. 5.4.10 Pre-existing Health Status of the Marshallese The general health status of both exposed and non-exposed inhabitants could have confounded interpretation of epidemiological data. Early surveys of the island community demonstrated unsanitary conditions with regards to flies, garbage disposal, and excretory habits, which made for multiple parasitic infestations and diseases in the population prior to their exposure. After the Bravo detonation, there were numerous serious epidemic diseases, such as poliomyelitis, influenza, chicken pox, and pertussis. The extent to which such diverse clinical conditions could have modified the health impact of radiation exposure, or affected interpretation of radiobiological data is unknown. 36