<P> The conservation tasks (and hence Piaget's theory) have been criticized on a number of fronts in regards to research methods . Many studies have looked at variations of the conservation tasks and how these variations affect children's responses . For example, studies show that children need to be assessed both verbally and non-verbally, as assessing them solely in a verbal manner can lead test results indicating that a proportion of the children are unable to conserve, while in actuality some children are only able to answer conservation tasks correctly in a non-verbal manner . </P> <P> Research has suggested that asking the same question twice leads young children to change their answer as they assume that they are being asked again because they got it wrong first time around . The importance of context was also emphasized by researchers who altered the task so that a' naughty teddy' changed the array rather than an experimenter themselves . This seemed to give children a clear reason for the second question being asked, and four - year - old children were able to demonstrate knowledge of the conservation of matter much earlier than Piaget's reported 7 - to 11 - year - old threshold for concrete operations . </P> <P> Research has also examined whether non-human primates are able to conserve . Chimpanzees are able to make judgements on whether two quantities of liquid are the same or different, and they are able to conserve correctly when liquids are transformed based on inferences . They are also able to correctly conserve for solid quantities, but they are not able to complete conservation of number tasks . Orangutans are able to differentiate between same and different quantities of liquid but they are only able to pseudoconserve, similar to children in the second stage of conservation development, and ultimately fail to correctly complete conservation of liquid tasks (see Age of conservation ability). </P>

Define and differentiate preoperational and concrete operational stages