Classification of research design based on time dimension

Classification of research design based on time dimension
The designs can be classified based on when and how often data will be collected in a study. The data can be collected at a single point in time or more than one occasion. 

There are four situations in which it is a appropriate to design a study with multiple points of data collection.
1. Time related process- Depends on how phenomena evolved over time, for example, wound healing process.
2. Time sequenced phenomena- It is very important to ascertain the sequencing of phenomena. For example, the researcher has to make sure that pregnancy contributes to diabetes mellitus and not diabetes mellitus precedes pregnancy.
3. Comparative purposes- Here, multiple data points are used to compare phenomena over time.
4. Enhancement of research control- The researcher collects data at multiple points to enhance the interpretability of the results. For example, the collection of pre test data with demographic variables allows the researcher to detect for initial group difference. 

Cross sectional design
The researchers collect the data at one point in time and describe the status of phenomena or relationships among phenomena. For example, the researchers assess the level of depression among post partum women and test the relationship between independent (age, socio economic status and level of education) and dependent variables (level of depression). Advantages of cross sectional designs are economical and easy to conduct the studies for the researcher. A major disadvantage of conducting a cross sectional study is that problems arise when society changes rapidly. The researcher can not generalize the cross sectional study findings to a rapid changing society.
Longitudinal design
In this design, the researcher collects the data over an extended time period, identifies changes over time and sequence phenomena like a chain, which is an essential to establish causality.
Longitudinal studies are three types;

1. Trend studies
2. Panel studies
3. Follow up studies

Trend studies
Trend studies help the researcher to collect the data by drawing samples from general population over time with respect to a phenomenon. The researcher draws different samples from same population at repeated intervals, examines the changes at present and makes predictions about future. Foe example,predicting nursing shortage in coming years.
Panel studies
The researcher collects the data from the same participants at different period of time. With the same participants, the researcher examines the changes and identify the reasons fro change. The major problem with the panel studies are subject attrition. Subject attrition is defined as dropping of study participants during the study.
Follow up studies
In follow up studies, the researcher collects the data subsequently, especially after an intervention. In non experimental studies, the sample may be followed periodically to assess the changes in the variables. For example, weight gain among premature infants.