Review

Use of group-randomized trials in pet

population research

L.K. Lord a,*, T.E. Wittum a, J.M. Scarlett b

a Department of Veterinary Preventive Medicine, College of Veterinary Medicine,

The Ohio State University, 1920 Coffey Road, Columbus, OH 43210, United Statesb Department of Population Medicine and Diagnostic Sciences, College of Veterinary Medicine,

Cornell University, Ithaca, NY 14853, United States

Received 10 March 2007; received in revised form 7 June 2007; accepted 10 July 2007

Abstract

Communities invest considerable resources to address the animal welfare and public health concerns

resulting from unwanted pet animals. Traditionally, research in this area has enumerated the pet-owning

population, described pet population dynamics in individual communities, and estimated national eutha-

nasia figures. Recent research has investigated the human–animal bond and explored the community

implications of managed feral cat colonies. These reports have utilized traditional epidemiologic study

designs to generate observational data to describe populations and measure associations. However, rigorous

scientific evaluations of potential interventions at the group level have been lacking. Group-randomized

trials have been used extensively in public health research to evaluate interventions that change a

population’s behavior, not just the behavior of selected individuals. We briefly describe the strengths

and limitations of group-randomized trials as they are used to evaluate interventions that promote social and

behavioral changes in the human public health field. We extend these examples to suggest the appropriate

application of group-randomized trials for pet population dynamics research.

# 2007 Elsevier B.V. All rights reserved.

Keywords: Group-randomized trial; Pet overpopulation; Study methodology

1. Introduction

Unwanted pets are a widely recognized problem in the United States that is associated with

considerable animal welfare and public health concerns. The common means by which

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Preventive Veterinary Medicine 82 (2007) 167–175

* Corresponding author. Tel.: +1 614 292 1206; fax: +1 614 292 4142.

E-mail address: lord.19@osu.edu (L.K. Lord).

0167-5877/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.prevetmed.2007.07.007

communities manage the problem of unwanted pets is with animal shelters. Shelters serve to

reunite owners with lost pets, find unclaimed animals new homes, or euthanize unwanted

animals. Today, there is a major movement among animal sheltering groups to reduce or

eliminate the euthanasia of healthy, adoptable animals in shelters and to identify means to

increase adoptions (Rowan and Williams, 1987). It appears that some success in reducing

euthanasia in shelters has been achieved. Annual euthanasia numbers have declined from an

estimated 13.5–18.6 million dogs and cats in the 1970s to the most recent national estimates of

4–6 million dogs and cats (Rowan and Williams, 1987; Nassar et al., 1992; Arkow, 1994; Irwin,

2001). Despite this progress, a large number of unwanted dogs and cats continue to be

euthanized in shelters each year, demanding further investment of resources and need for better

solutions.

The academic research community has also invested resources to better understand pet

population dynamics. In the 1970s, the major focus of research was on the basics of trying to

enumerate the pet-owning population, describe the pet population dynamics in several

communities, and estimate national euthanasia figures (Schneider and Vaida, 1970; Franti and

Kraus, 1974; Griffiths and Brenner, 1976; Nassar and Mosier, 1980, 1982, 1986; Nassar et al.,

1984). National humane groups such as the American Humane Association and the Humane

Society of the United States also invested resources in trying to estimate the number of animal

shelters and national euthanasia figures (Rowan and Williams, 1987; American Humane

Association, 1992). These early efforts faced many obstacles, primarily the lack of a census for

the pet-owning community and the lack of an accurate list for the number of animal shelters in the

United States. These problems remain today.

Since the 1990s, the focus of pet population dynamic research (called by many ‘‘pet

overpopulation’’ research) has shifted to attempt to better understand reasons for owner

relinquishment of pets resulting from the breakdown of the human–animal bond, factors

associated with successful adoptions, evaluation of the health effects of early age spay-neuter

and the implications of the increasing feral cat population (Patronek et al., 1996a,b; Salman

et al., 1998, 2000; Scarlett et al., 1999; New et al., 2000; Hughes et al., 2002; Hughes and Slater,

2002; Levy et al., 2003; Spain et al., 2004a,b). Much of this research has utilized basic

epidemiological study designs including cross-sectional studies, case–control studies, and

retrospective cohort studies. This era of research has made a tremendous contribution toward a

better understanding of the factors associated with overpopulation and has laid the groundwork

for further advancement.

Although basic epidemiologic data have been generated and reported, studies of population

interventions to reduce pet overpopulation are sparse. Few reports of rigorous scientific

evaluation of interventions such as the use of individual randomized trials and group-randomized

trials have been reported. Just as individual randomized trials are considered the ‘‘gold standard’’

for studies to evaluate interventions at the individual level, group-randomized trials (GRT) are

considered the ‘‘gold standard’’ for evaluation of interventions delivered at the group level.

Group-randomized trials have been used extensively in public health research to evaluate

interventions that change a population’s behavior not just the behavior of selected individuals.

The value of group-randomized trials is their effectiveness in evaluating population-level

strategies that seek to change the social norm for a particular behavior. Although increasing the

use of individual randomized trials would be of benefit in pet population research, their

techniques are well described in the veterinary literature and will not be discussed further in this

paper. Despite being used commonly in public health research, group randomized trials have not

been utilized in companion animal research and will be the focus in this manuscript.

L.K. Lord et al. / Preventive Veterinary Medicine 82 (2007) 167–175168

The goals of this report are to: (1) briefly describe how group-randomized trials are used to

evaluate interventions that promote social and behavioral changes in the human public health

field, including a discussion of several of the biases and other issues that affect these trials; (2)

provide examples of how group-randomized trials may be used in pet population dynamics

research. While it is not our intention to provide a comprehensive review of the literature

regarding the use of group-randomized trials, we hope that this manuscript will serve to stimulate

interest and discussion regarding the application of this methodology to the study of pet

overpopulation.

2. Use of group-randomized trials in public health research

2.1. Introduction to group-randomized trials

A group-randomized trial (GRT) is a study designed to evaluate an intervention that operates

at a group level such as a school, a work-site or a community, changes the social or physical

environment, or cannot be delivered at the individual level (Murray, 1998; Donner and Klar,

2000). These studies have been advocated in public health research over the last 30 years to

achieve widespread change in health behaviors. Common groups used in these studies include

communities, worksites and schools. GRTs are considered the gold standard when the allocation

of interventions to groups and not to individuals is necessary.

There are several characteristics, which make group-randomized trials different from the

standard randomized clinical trials used in clinical medicine (Murray, 1998; Murray et al., 2004).

The unit of assignment in a GRT is a group rather than an individual, and the group adds an extra

level of nesting and variability within the study. Groups are also not formed at random but are

based on some common characteristic related to social structure, geography or other common

characteristic. Also, due to the complexity and size of groups in most GRTs, only a small number

of groups can be allocated to the intervention and control conditions, and often the size of the

groups may be relatively large (hundreds/group). Whereas a single drug or treatment is often

evaluated in a clinical trial, interventions in a GRT have traditionally been multi-factorial and

have involved a variety of educational programs, mass media campaigns, and social marketing

tools designed to influence group behavior.

There are numerous examples of group-randomized trials in the public health literature. In the

1970s three large community intervention trials, the Stanford Five-City Project, Minnesota Heart

Health Project, and Pawtucket Heart Health Project, were funded by the National Heart, Lung

and Blood Institute that targeted reducing risk factors for coronary heart disease such as high

blood pressure, elevated cholesterol, and cigarette smoking (Farquhar et al., 1985; Luepker et al.,

1994; Carleton et al., 1995). None of these early studies included randomization of the groups,

and magnitude and level of significance for reduction of risk factors were mixed in the three

studies. In 1989, the National Cancer Institute funded the Community Intervention Trial for

Smoking Cessation (COMMIT) (COMMIT Research Group, 1991; Green et al., 1995). In this

study, 11 matched pairs of communities were randomized to evaluate a 4-year intervention to

encourage smokers to quit smoking. Significant declines in smoking prevalence were seen among

light to moderate smokers, particularly those in the less educated group. In another study,

Communities Mobilizing for Change on Alcohol (CMCA), 15 communities were randomized to

evaluate a community-organizing intervention to limit teen access to alcohol (Wagenaar et al.,

2000). A decline in arrest and traffic crash indicators was found in the intervention communities

as a result. Group-randomized trials have also been used in both the workplace and school setting

L.K. Lord et al. / Preventive Veterinary Medicine 82 (2007) 167–175 169

to promote healthy behaviors such as physical activity and improved eating habits, and to

discourage risky behaviors such as smoking (Secker-Walker et al., 2006; Boutelle et al., 2001;

Stevens et al., 2005).

2.2. Potential sources of bias in group-randomized trials

GRTs are particularly susceptible to several forms of bias that must be controlled for as much

as possible (Murray, 1998, 2001). The first type of bias is selection bias, meaning that differences

exist between groups before the intervention is carried out and these differences are not evenly

distributed between treatment and control groups. In most individual randomized trials this bias

is handled through randomization. In GRTs, however, with the small number of groups it may not

be possible to completely eliminate selection bias even with randomization. Selection bias is part

of the reason that having as many groups as possible is critical in GRTs.

Group-randomized trials occur in the ‘‘real’’ world, and schools, worksites and communities are

potentially exposed to influences other than the intervention. If these other external influences have

an impact on the entire population in the study, they are referred to as bias due to history. If these

influences equally affect both treatment and control groups, they are less of a concern than those that

only affect groups in one arm of the study, the latter being called bias due to differential history. For

example, if the intervention involves implementing a smoking cessation program in the treatment

communities, bias due to differential history would occur if during the study period the local health

departments in some of the control communities also implemented a smoking cessation campaign.

This could erroneously cause the investigators to conclude the intervention had less of an effect in

reducing smoking in the treatment communities than was actually the case.

Another type of bias that is of particular concern in GRTs is maturation bias, also known as

secular trends. This has been a concern in some of the more recent community trials such as the

coronary heart disease trials and COMMIT discussed above. This type of bias occurs when the

natural development or long-term trends of a behavior affect the results of the study, and can be

non-differential or differential. For example, in the COMMIT study it was believed that the

secular trend of a decline in smoking among the control groups masked part of the intervention

effect found in the intervention groups. Although secular trends cannot be prevented, study

design techniques can be implemented to help evaluate and control for their effects.

The other major type of bias that is of particular concern in GRTs is that of contamination.

This occurs when the program that is implemented in the intervention groups leaks or crosses

over into the control groups. For example, this can occur when community leaders in one or

more of the intervention groups share intervention information with their counterparts in

control groups, and programs suddenly appear in the control groups. Researchers must take

particular care in monitoring the control groups for contamination and making sure that

participants in the intervention groups understand the need to keep program information in their

own groups.

2.3. Other methodological issues in group-randomized trials

One major issue with group-randomized trials that is difficult to address despite an adequate

design is allowing enough time for the intervention to take effect. In many cases, an intervention

that involves changing behavior can take years to have its full impact, and depending on the

amount of time and funding available, evaluation of the intervention may occur prior to its

reaching its full impact.

L.K. Lord et al. / Preventive Veterinary Medicine 82 (2007) 167–175170

Several advances have been made in the design of these studies, particularly in community-

based trials (Sorensen et al., 1998; Thompson et al., 2003). One of the areas that researchers have

tried to address is community involvement. In early intervention trials, the intervention was

chosen by the researchers with no input from the community, while today most studies include a

community advisory board (Sorensen et al., 1998; Thompson et al., 2003). Most community-

based trials also include some type of process evaluation to understand how the components of

the intervention are delivered and to make sure they are received by individuals. Finally, more

emphasis has been put on maintenance of intervention programs, particularly if they are

successful. Researchers, groups and funding organizations need to consider what is required to

maintain a successful intervention program at some level in order to both monitor long-term

trends beyond the initial study and to help ensure community buy-in of the program.

2.4. Statistical issues in group-randomized trials

There are several issues that must be accounted for in GRTs that are not of issue in individual

randomized trials. First, members of groups have something in common, and this level of

correlation (called the intraclass correlation coefficient or ICC) must be taken into account (Kish,

1965). The ICC is almost always positive in group-randomized trials and leads to an increase in

the variance of the group means. This ICC lowers the power of a study and if not accounted for in

the analysis, will inflate the type I error rate (Feng et al., 2001; Murray, 1998). Second, because

the unit of assignment is the group and not the individual, testing for intervention effects depend

on the degrees of freedom based on the number of groups. Because the number of groups is small,

the power of these tests for intervention effects is further reduced and if the incorrect degrees of

freedom are used, the type I error rate will also be inflated. This can be particularly problematic in

detecting statistically significant intervention effects in group-randomized trials. It has also been

shown that by modeling time explicitly in the analysis and adjusting for individual level

regression covariates, the operative ICC in group-randomized trials can be reduced (Murray and

Blitstein, 2003).

3. Using group-randomized trials in pet population research

Many proposed programs to reduce pet overpopulation are targeted at groups and cannot be

effectively evaluated at the individual animal or owner level. These programs are implemented at

a larger level such as communities and are often designed to initiate societal change. Thus the use

of group-randomized trials is warranted to provide rigorous scientific evaluation of their effect.

Below are examples of how group-randomized trials could be used in pet population research.

3.1. Evaluation of feral cat trap-neuter-return (TNR) programs

An issue of importance that lends itself to evaluation with group-randomized trials is the use

of trap-neuter-return (TNR) programs to reduce the feral cat population in communities. The

problem of free-roaming cats has been well documented, and is one of the largest challenges

facing the humane movement in the 21st century (Patronek and Rowan, 1995; Kirkwood, 1998;

APPMA, 2006). TNR programs are largely promoted to help address this growing problem, but

few rigorous studies have been done to evaluate their effectiveness. One descriptive study showed

a decline in the percentage of impounded cats euthanized and an increase in the percentage of

impounded cats adopted after a TNR program was implemented in a community (Hughes et al.,

L.K. Lord et al. / Preventive Veterinary Medicine 82 (2007) 167–175 171

2002). The authors recognized, however, that this effect could not be attributed directly to the

TNR program, as there were other changes implemented during the same time in the community

that could also have explained the observed effects. Two studies conducted on university

campuses showed a decline in the number of cats and kittens caught when a comprehensive TNR

and adoption program was implemented (Hughes and Slater, 2002; Levy et al., 2003). These

latter two studies demonstrate that in smaller, controlled environments with active caregivers,

TNR could be an effective means to control and even reduce feral cat populations. In a more

recent study using mathematical population modeling, TNR was not shown to reduce the per

capita growth in cat populations, and any population effects were minimal for two large county

TNR programs (Foley et al., 2005). In another study that employed matrix population models, it

was shown that annual neutering of greater than 75% of the fertile cat population would be

needed to control cat populations, which may be unrealistic for many larger scale TNR programs

(Andersen et al., 2004). These types of studies using mathematical modeling rely on certain

assumptions such as controlled populations and can make their findings somewhat difficult to

interpret or apply in a real-life setting. All of these studies, however, underscore the need for

further investigation to access the community impact of TNR programs, particularly as they

relate to reducing the numbers of cats entering shelters.

A group-randomized trial could be used to evaluate the effectiveness of TNR programs at the

community level. To set up a study, neighborhoods serviced by a particular humane society could

be defined by geographical boundaries using U.S. Census Bureau census tracts. These

neighborhoods could then be randomized into treatment and control areas, where treatment

neighborhoods would implement TNR programs for a specified time period and control

neighborhoods would not. Assessments of the number of feral cats and the number of complaint

calls at the humane society before and after the start of the intervention would be measured. The

humane society would track the addresses of all incoming feral cats to attribute them to their

respective neighborhoods. This approach would allow an area to be divided into neighborhoods

that were small enough for the intervention to have a realistic opportunity to have an impact,

instead of trying to measure the effect for an overall county as has been done in prior research.

3.2. Evaluation of programs to improve pet reunification

Reunification of pets is an important means to expeditiously move animals out of shelters.

Three epidemiological studies were conducted recently in Dayton, Ohio to describe the lost and

found search process (Lord et al., 2007a,b,c). Two initial studies used a cohort of lost dogs and

cats, respectively, that were identified by owner reports of losing a pet to local animal agencies or

placement of a lost pet advertisement in the local newspaper. These studies showed that only 48%

of dogs and 19% of cats were wearing some type of identification or had a microchip. Further, the

studies showed that for dogs, utilization of the animal sheltering system through calling and/or

visiting an animal agency, the pet wearing a dog license tag, and the hanging of neighborhood

signs all increased the probability of reunification. For cats, two-thirds returned home on their

own and the most successful method of reunification was posting of neighborhood signs. These

findings provide insight into the lost and found pet search process and point out many areas where

interventions through education on search methods and identification could be evaluated with

more sophisticated study designs. If these interventions were offered in communities or other

sites such as veterinary clinics, then the GRT design could be used.

For example, we know that in a population of dogs and cats that were lost, only 8% of dogs and

7% of cats had a microchip, and only 1.5% of the dogs and no cats that were reunited were done

L.K. Lord et al. / Preventive Veterinary Medicine 82 (2007) 167–175172

so by a microchip (Lord et al., 2007a,b). Given the emphasis placed on microchips as a recovery

method for lost pets by the humane community, particularly with the recent difficulties in

reunification during the response to hurricane Katrina, evaluation of methods to increase the use

of microchips is warranted. A community based group randomized trial could be used to evaluate

promotional programs in communities (intervention group) compared to control communities by

using pre- and post-intervention surveys of residents to evaluate if microchip promotion

increased their prevalence at the community level. Microchip and identification campaigns could

also be used in a similar approach to assess the shelter return to owner rate in those communities

where programs were implemented. Because these microchip and identification campaigns

would be offered at the community level and not to particular individuals, a group-randomized

trial would be required.

4. Conclusions

Pet population research would benefit from the use of group-randomized trials. Funding

agencies have invested considerable resources in programs to reduce pet overpopulation, but in

most cases scientific evaluation of the impact of these programs has not been conducted. Because

much of the funding is implemented through community groups such as humane societies,

funding agencies are increasingly asking what impact their money is having at the community

level. Funding groups, communities, owners and most importantly pets, will be better served if

community programs are scientifically evaluated using GRTs.

These types of interventional studies are not without challenges. They require a change in

approach and a redirection of research dollars. They will be expensive and time-consuming

and will require pilot studies to evaluate their feasibility and to estimate intraclass correlation

coefficients so that larger studies have sufficient statistical power. Traditional epidemiologists

will have to involve researchers with expertise in the field of health behavior and promotion

and be willing to devote resources to establishing community involvement. This approach

may appear daunting and require more resources than traditional funding agencies are willing

to provide. As a society, however, the continued euthanasia of millions of pets in animal

shelters in the United States each year is morally unacceptable and as researchers, we have a

responsibility to advance our knowledge to impact the problem to the greatest extent

possible. We ask that the research community and funding agencies consider the need for

better evaluation of interventions and explore the use of group-randomized trials as a method

to do so.

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