The model consists of at least three groups of people: unbelievers, enthusiasts who alone are responsible for spreading the faith, and inactive believers. If demographic effects are included then unbelievers are split into those open to conversion and those hardened against. See Limited Enthusiasm and Demographics for full details of the core model and dynamic hypotheses
The migration model is built on top of the limited enthusiasm model with demographics. Thus growth is driven by enthusiasts making converts and enthusiasts from those converts. In addition the children of believers may join the church at a later stage, some as enthusiasts.
The rationale for the model is that most churches maintain some form of membership with varying degrees of criteria which reflects who "belong" to the church and who do not. Generally this is not the same as the attendance at the church. There are people who attend who chose not to belong. Likewise there are people who belong to the church who no longer attend.
The additional hypotheses are:
Limited Enthusiasm with Membership
The central hypothesis of the limited enthusiasm model is that conversion growth in the church is driven by a sub-group of church members called enthusiasts. The model is extended to include membership as an additional variable to attendance/affiliation. Believers may become members through birth or conversion with different delays in taking membership. Likewise there are delays in removing people from membership when they die or leave.
The model predicts that in general attendance is higher than membership in a growing church, and lower than attendance in a declining church. This is a result of delays in joining and leaving alone, before any account is taken of attendees who never join the church, or members who attend infrequently.
|Converts Membership Delay||Converts who start attending the church do not become members immediately. There is a delay before they join, perhaps because there are membership classes, or they need time to think of the implications of membership, or the church is slow to update lists.|
|Believers' Children Membership Delay||Those who are born to believers, who then go on to join the church when older, become members after they are counted as attendees. The extent of this delay depends on what age the church starts counting people as attendees, the age they allow people to become members, and the fact that many young people temporarily drop out of church before they return and then make a commitment. The model does not handle churches who count young people as members even when it does not count them as old enough for attendance.|
|Resignation Delay||People who stop attending the church are not immediately taken off membership. This reflects the reluctance of churches to remove people who have left, hoping that they will return. It also reflects the fact that churches may not update their membership lists quickly. It also reflects that some people who stop attending do so gradually by reducing the frequency of attendance.|
|Deceased Members Delay||When people die they may not be immediately removed from membership. churches only update lists annually at best, often longer. Sometimes people have stopped attending before death due to infirmity.|
In addition to the Limited Enthusiasm and Demographics flows, there are four coflows connected to these models and derived from the four above assumptions for the membership stock. All are pipeline delayed except for those born into the church which is smooth, reflecting its longer timescale and the possible errors this introduces.
Because of these delays membership will always lag that of attendance. This is an inevitable result of the assumptions and the SD structure. The model investigates the extent of the delay, and the difference between attendance and membership at any time.
Generally gap between attendance and membership is an indicator of the health of the church. Growing churches will have a positive attendance/membership gap. Declining churches have a negative gap.