The strange situation: a sequence of events where the child and a parent are brought into a room with toys. Subsequently, a stranger is introduced who talks to the parent, after which the parent leaves. After a while, the parent enters again, and both the parent and the stranger leave. Some time passes and the stranger enters again. Finally, the parent re-enters the room as well. Throughout all this, the child's behavior is observed. This technique is used less and less.
Attachment Q-Sort (AQS): a more recently developed list of 90 behaviors that the researcher brings to the house of the child, where several observation sessions will take place.

Four Types

Based upon this research, four types of attachment have been distinguished in children:

Safely attached children: they use their caretaker as base to explore new surroundings and often return to the caretaker for support. They cry when the parent leaves, but are quickly comforted when he or she returns. About 60 – 70% of the children are attached this way.
Avoiding children: these children respond minimally to the parent and do not cry when he or she leaves the room. Their reaction to a stranger is similar as their reaction to a parent. When reunited they avoid contact with their parent, or respond very slowly to the greeting. About 20% of children shows this attachment type.
Scared or recalcitrant children: cling to the parent when brought into a new room, and do not explore their new environment. They are absolutely shocked when the parent leaves. When he or she returns, the child responds with a combination of clinging and aggression. Notoriously hard to comfort. About 10 – 15% of children fall into this type.
Disorganized and disoriented children: when reunited with the parent that left, these children are confused and respond with opposite behavior types. They look away when hugged by their parents and do not initiate contact. This type of attachment occurs in 5 – 10% of the children.

Correlations have been found between any of the three types of unsafe attachment and the quality of social relationships later in life. However, it is important to realize that a correlation does not necessarily imply a causal connection.

References

Van IJzendoorn, M.H.; Vereijken, C.M.J.L.; Bakermans-Kranenburg, M.J. & Riksen-Walraven, J.M. (2004). Assessing Attachment Security With the Attachment Q-Sort= Meta-Analytic Evidence for the Validity of the Observer AQS. Child Development. 75(4), pp. 1188 – 1213.
Rees, C. (2008). Children’s attachments. Paediatrics and Child Health. 18(5), pp. 219 – 226.