This document summarizes statistical analysis techniques for assessing the validity of survey results on violence against women. It discusses how stratified analysis and multivariate logistic regression can be used to control for confounding variables. An example is provided showing how multivariate analysis revealed that poverty, urban residence, and high parity independently increase women's risk of violence. The document also describes how more advanced techniques like life table analysis can provide additional insights, such as that high parity may result from violence rather than be a risk factor for it.

1. Y C H A P T E R T W E LV E
FIGURE 12.9 HUSBAND’S CHARACTERISTICS ACCORDING TO USE OF VIOLENCE
which variations in a specific outcome (for
(AS REPORTED BY WIFE) IN CENTRAL JAVA, INDONESIA example, the risk of violence) may be
attributed to variations in an independent
Husband characteristics variable (for example women’s age or edu-
Alcohol use 3** cation). External validity refers to the
13
degree to which results from a given study
Son preference 20** may be used to draw conclusions about a
30
larger population. If a study is performed
42**
Low education on a randomly selected population, it
49
2**
should be possible to generalize the results
Fights with men
6 of the study to the general population from
Has other women 2** which the sample was drawn.
13
Another important question is Are the
0 10 20 30 40 50 60
findings consistent? That is, do they make
Percent of men
sense, according to what is known about
I Not violent I Violent the subject locally and internationally? If
Percentages are given for the proportion of men with each characteristic, they differ greatly from previous findings,
according to whether or not they have ever used physical or sexual violence are there any additional data to support the
against their wives.5 new results? Are there any aspects of data
(From Hakimi et al, 2002.5) ** p<.001 N=765 men collection, sampling, design, or analysis
that might have altered the results by intro-
Indonesian women who had ever experi- ducing bias? Has the analysis taken into
enced partner violence reported more account possible sources of confounding?
recent symptoms of ill health such as pain, The following pages present different ways
dizziness, ulcers, and intestinal problems. to address these issues.
Figure 12.9 shows that Indonesian men
who have been violent towards their wives The effects of confounding in
are also more likely to have used alcohol, data analysis
and to have had extra-marital relationships. In studying the association between risk
They are more likely to have been factors and a specific problem, confound-
involved in fights with other men, to prefer ing can occur when another characteristic
sons over daughters, and to have little or exists in the study population and is asso-
no education. ciated with both the problem and the risk
factor under study.
A S S E S S I N G T H E VA L I D I T Y Confounding can have a very impor-
O F S U R V E Y R E S U LT S tant effect on study results, and can cre-
ate the appearance of a cause-effect
Once you have found what seem to be the relationship that in reality does not exist.
most important results from your data, and Age and social class are often con-
you have performed basic statistical tests founders in epidemiological studies. In
between variables (for example, violence the study of risk factors for violence, con-
and ill health), you need to assess their founding variables can give misleading
validity. This means you need to deter- impressions about what risk factors influ-
mine to what degree the study measured ence the occurrence of violence.
what it was supposed to, and whether the Stratified analysis and multivariate
findings mean what they are supposed to. analysis are two ways to control for the
Internal validity refers to the extent to effects of confounding variables.
194 Researching Violence Against Women

2. A N A L Y Z I N G Q U A N T I TAT I V E D ATA
Y
Stratification involves analyzing data TABLE 12.3 PREVALENCE OF EMOTIONAL DISTRESS ACCORDING TO
separately using defined categories of the MARITAL STATUS AMONG NICARAGUAN WOMEN
confounding factor, such as age groups.
Selection of women Percentage of emotional distress
For example, some studies using bivari-
All women 15–49 (n=488) 17%
ate analysis (analysis using only two
Ever-married women (n=360) 20%
variables) have found that pregnant
Never-married women (n=128) 10%
women were more likely to be abused
than non-pregnant women. However, (From Ellsberg et al, 1999.6)
after analyzing the same data stratified by
age groups, it turned out that this associa- TABLE 12.4 PREVALENCE OF EMOTIONAL DISTRESS AMONG EVER-MARRIED
tion was confounded by age. It turned NICARAGUAN WOMEN ACCORDING TO EXPERIENCES OF WIFE ABUSE
out that being young was the real risk
Experience of wife abuse Prevalence of emotional distress
factor for violence rather than pregnancy.
Never abused (n=172) 7%
It just happened that younger women
Ever abused (n=188) 31%
were more likely to be pregnant than
older women. This explained the (From Ellsberg et al, 1999.6)
increased prevalence of violence among
pregnant women. FIGURE 12.10 THE CONFOUNDING EFFECT OF VIOLENCE ON THE
Violence can also be analyzed as a con- ASSOCIATION BETWEEN MARRIAGE AND MENTAL DISTRESS
founding variable for other risk factors, as
shown in the following example of a study
Marriage
on mental distress. Preliminary results
found that women who had been married
at least once in their lives had twice as
Violence
much emotional distress as women who
had never been married (Table 12.3). This
would imply that marriage is an important
risk factor for mental distress. Mental Distress
However, when the prevalence of mental
distress among ever-married women was
analyzed separately according to whether
women had experienced wife abuse, a abuse and not marriage itself that accounts
large difference was found between the for the increase in mental distress among
two groups. Thirty-one percent of abused married women.
women suffered mental distress, compared This analysis is further strengthened by
to only seven percent of women who had comparing women’s current mental dis-
never been abused, which is even less than tress according to the severity of violence
the prevalence of distress among never- they experienced and when it took place,
married women (Table 12.4). as shown in Figure 12.11. Breaking down
Since wife abuse is associated with mar- the analysis this way demonstrates that
riage (by definition only ever-partnered women who were severely abused in the
women can experience wife abuse) and it last 12 months were over ten times more
is also associated with mental distress, it likely to be distressed than women who
has a confounding effect on the association had never been abused. Further, it
between marriage and mental distress revealed that the severity of abuse
(Figure 12.10). Therefore, after stratified was more important than when it took
analysis it becomes evident that it is wife place, since women experiencing severe
A Practical Guide for Researchers and Activists 195

3. Y C H A P T E R T W E LV E
FIGURE 12.11 PREVALENCE OF EMOTIONAL DISTRESS ACCORDING TO
EXPERIENCES OF VIOLENCE AMONG NICARAGUAN WOMEN
Emotional Distress %
10 20 30 40 50 Crude Odds Ratios 95% Confidence Interval
Experiences of
Violence
No violence 7% 1.0
Former moderate 15% 2.3 .7–7.8
21% 1.0 –12.0
Current moderate 3.6
Former severe 27%
4.8 2.2–10.8
Current severe 44%
10.3 4.9–21.6
Percentages are given for the proportion of ever-married women who experienced emotional distress in
the four weeks prior to the survey, according to whether they had experienced physical partner violence.
Violence was classified by severity and by whether it took place within the 12 months previous to the
study, or earlier. In the right hand columns, crude (unadjusted) odds ratios and their corresponding confi-
dence intervals are given. Intervals where the lower and upper figures do not include 1.0 are considered
statistically significant. (In this case, all types of violence except for former moderate violence are signifi-
cantly associated with emotional distress.)
violence formerly were still more likely to that urban women and women with many
be currently distressed than women who children are found to have greater levels
had suffered only minor abuse, even of violence, simply because they are more
though it took place more recently. likely to be poor? How can we unravel
the complex relationships between these
The use of multivariate analysis to variables?
adjust for confounding factors Multivariate analysis techniques, such
When it appears that there are several as logistic regression modeling, are
variables confounding an association, then useful for examining the relationships
it is no longer practical to use stratified between several explanatory factors and a
analysis, as it would be excessively com- specific outcome variable. Logistic
plex to perform. For example, in Figure regression helps to uncover the degree
12.7, we saw that in the León study there to which several explanatory variables are
were several variables, such as poverty, related and to control for confounding
living in the urban area, and number of variables. In Table 12.5 (next page), the
children, which were associated with the same relationships presented in Figure
risk of violence. It was further shown that 12.7 are examined using crude or unad-
these three variables are associated with justed odds ratios as well as multivariate
each other as well as with women’s level or adjusted odds ratios. The 95 percent
of education. Could it be that poverty is confidence intervals are used to assess
the true underlying factor influencing the statistical significance of the associa-
women’s risk of violence, and therefore tion by indicating that there is a 95
196 Researching Violence Against Women

4. A N A L Y Z I N G Q U A N T I TAT I V E D ATA
Y
percent probability that the true figure TABLE 12.5 ASSOCIATIONS BETWEEN BACKGROUND FACTORS
lies between this range. If the range AND PREVALENCE OF VIOLENCE AMONG 360 EVER-MARRIED
NICARAGUAN WOMEN AGES 15–49
between the lower and upper figure in
the confidence interval does not include Crude Adjusted
one, then it can be said that there is a 95 Variable Categories OR (95% CI) OR (95% CI)
Poverty Nonpoor 1.0 1.0
percent probability that the association is Poor 1.91 (1.12-–3.23) 1.82 (1.03–3.23)
not due to chance. Zone Rural 1.0 1.0
When comparing the crude and multi- Urban 1.62 (.94-–2.78) 2.07 (1.12–3.82)
variate odds ratios for each variable, one Number of 0–1 1.0 1.0
can see that they do not vary much in children 2–3 1.40 (.82-–2.39) 1.34 (.74–-2.43)
most of the cases. The association 4 or more 2.77 (1.59-–4.82) 2.23 (1.21–-4.15)
between violence and poverty, having Family No history
history in wife’s family 1.0 1.0
more than four children, and a history of
of abuse Wife’s mother
family violence in the husband’s family abused 1.8 (1.24-–2.90) 1.28 (.79-–2.09)
are all maintained. Living in the urban No history in
area, which had a confidence interval husband’s family 1.0 1.0
slightly below one in the crude analysis, Husband’s mother
abused 3.13 (2.00-–4.96) 2.98 (1.86–4.73)
becomes significant in the multivariate
model, while a history of family violence Crude and adjusted odds ratios are given (together with 95 percent confi-
in the wife’s family becomes insignificant. dence intervals) for having experienced violence at least once in their lives.
After performing the multivariate analy- (From Ellsberg et al, 1999.4)
sis, it is possible to say that although
poverty, urban/rural residence, and high Using advanced statistical
parity are all related, their effect on analysis creatively
women’s risk of violence is independent In earlier sections of this chapter, we pre-
and should not be interpreted as the sented the most commonly used tech-
result of confounding. niques for statistical analysis of survey data
FIGURE 12.12 TIME FROM THE START OF A RELATIONSHIP TO THE ONSET OF VIOLENCE
Percentage
of women
100–
80–
60–
40–
20–
0–
0 2 4 6 8 10 12 14 16 18 20 22
Number of years from start of marriage to onset of violence
The figure shows the cumulative incidence of domestic violence over time among 188 women who reported
having experienced marital violence at least once in their lives, using Kaplan Meier Life Table Analysis.8
(From Ellsberg et al, 2000.8)
A Practical Guide for Researchers and Activists 197

5. Y C H A P T E R T W E LV E
FIGURE 12.13 THE PROBABILITY OF LEAVING AN ABUSIVE RELATIONSHIP OVER TIME,
BASED ON A WOMAN’S CURRENT AGE GROUP
Percentage of
women leaving
100–
90–
80–
70–
60–
50–
40–
30–
20–
10–
0–
0 2 4 6 8 10 12 14 16 18 20 22 24 26
Number of years from onset of violence to leaving
15–24 25–34 35–49
p<.01
This figure shows that 50% of young women (ages 15–25) leave a relationship within four years of
violence starting, whereas 20 years was the median time of older women.
(From Ellsberg, 2000.10)
on violence. However, additional insight implies that high parity, instead of being
may be revealed by the creative use of a risk factor for abuse, is more likely to
more advanced statistical techniques. be a result of violence, because battered
For example, life table or survival women are less likely to be able to con-
analysis was used to gain a deeper trol the timing of sex or the use of birth
understanding of the relationship between control.
violence and high parity in Nicaragua. The same techniques were applied to
Many international studies have found a the likelihood of a woman leaving an
similar association.7 One interpretation for abusive relationship, and it was found
this is that having many children places that 70 percent of women eventually did
additional stress on a marriage and leave their abusers, although some
increases a woman’s likelihood of being women stayed as long as 25 years or
beaten by her husband. more before separating. Stratifying this
However, using life table analysis, a analysis according to age groups shows
statistical technique which measures the that younger women are more likely to
probability of events occurring over time, have left an abusive relationship within
it was possible to determine that violence four years, compared to women
began early on in relationships, in many between 35-49 years (Figure 12.13). This
cases well before women had started indicates that younger women are less
bearing children. Figure 12.12 shows that likely to tolerate abuse than older
50 percent of violence begins within two women. In order to use survival analysis
years of marriage, while 80 percent of techniques, it is necessary to collect
abuse starts within four years. This detailed data regarding each of a
198 Researching Violence Against Women

6. A N A L Y Z I N G Q U A N T I TAT I V E D ATA
Y
woman’s relationships: when did it start, BOX 12.2 SUGGESTED GUIDELINES FOR WRITING A SCIENTIFIC PAPER
how long did it last, was there violence,
and if so, when did the first and last Abstract
incidents of violence take place. These Approximately 100 words.
types of analyses are somewhat compli-
Background
cated to perform and interpret, so it is Literature, national context, objectives.
important to consult with an experi-
enced statistician. Methods
Describe the study population, how the sample was selected, what instruments were
used, how the fieldwork was conducted, how data were analyzed, how ethical
INTERPRETING clearance was obtained, and any special measures, such as safety procedures.
T H E R E S U LT S
Results
This section should describe all the major results of data analysis, including relevant
The process of data analysis will often
tables and figures, and measures of statistical significance.
take longer than you initially expect.
However, you can plan data analysis in Discussion
stages, so that initial findings, such as The purpose of this section is to interpret the meaning of data, assessing the validity
and generalizability, possible sources of bias, how the findings relate to interna-
prevalence and descriptive characteristics,
tional and national studies on the same subject, and possible explanations for the
can be made available as soon as possible most important findings.
to the communities and local institutions
that have been supporting the research, Conclusions
These are sometimes included in the discussion section. How might these findings
and that will be anxiously awaiting results.
be used for improving interventions and policy? What are areas that might benefit
Further analysis can be performed over a from future research?
longer period to explore some of the more
interesting findings in greater depth. Box References
Make sure to include citations from the most relevant literature in the field of study.
12.2 presents guidelines for writing up
research results for publication in scientific
(From Persson and Wall, 2003.1)
journals. Chapter 14 will discuss in detail
how research results may be tailored to fit
the needs of different groups. two variables, but unless you have
When interpreting and writing up the good information about when different
results of data analysis, it is important to conditions or events occurred it is diffi-
be cautious. Each research design yields cult to know with certainty what came
different kinds of data, with their respec- first. A good example of how causal
tive limitations. Be careful not to draw relationships can be misinterpreted is
conclusions that are not supported by the the relationship between parity and vio-
data, as overstating your results can seri- lence presented in the last section. It is
ously undermine the credibility of the a good idea when presenting results
research. People are more likely to listen from cross-sectional surveys to talk
to your findings when you are open about “associations” rather than causes.
about whatever limitations the study had The discussion section can assess
in terms of design, data collection, or which variables are most likely to be
analysis. Some examples of common pit- causes or outcomes, based on your
falls are the following: conceptual framework and other stud-
ies on the subject.
I Inferring causal relationships from
cross-sectional data. Cross-sectional sur- I Inferring causal relationships from bi-
veys can highlight associations between variate analysis. As we showed in the
A Practical Guide for Researchers and Activists 199

7. Y C H A P T E R T W E LV E
example on marriage and emotional 1. Persson LÅ, Wall S. Epidemiology for Public
distress, other variables may confound Health. Umeå, Sweden: Umeå International
School of Public Health; 2003.
a relationship between two variables. If
2. Yoshihama M, Sorenson SB. Physical, sexual, and
you have not performed stratified or emotional abuse by male intimates: Experiences
multivariate analysis, it is wise to be of women in Japan. Violence and Victims.
cautious in interpreting your results. 1994;9(1):63-77.
3. Rosales J, Loaiza E, Primante D, et al. Encuesta
I Generalizing conclusions for different Nicaraguense de Demografia y Salud, 1998.
Managua, Nicaragua: Instituto Nacional de
populations than the study population.
Estadisticas y Censos, INEC; 1999.
Results that are representative for one 4. Ellsberg MC, Peña R, Herrera A, Liljestrand J,
region are not necessarily true for Winkvist A. Wife abuse among women of child-
other regions in the country, or for the bearing age in Nicaragua. American Journal of
country as a whole. This does not Public Health. 1999;89(2):241-244.
mean that regional studies cannot pro- 5. Hakimi M, Nur Hayati E, Ellsberg M, Winkvist A.
Silence for the Sake of Harmony: Domestic
vide important insights that are rele-
Violence and Health in Central Java, Indonesia.
vant for a much broader context. There Yogyakarta, Indonesia: Gadjah Mada University;
are many examples of regional studies 2002.
that made critical contributions for 6. Ellsberg M, Caldera T, Herrera A, Winkvist A,
guiding national policies and programs. Kullgren G. Domestic violence and emotional
However, it is still important to be distress among Nicaraguan women: Results from
a population-based study. American Psychologist.
careful in stating clearly what the limi-
1999;54(1):30-36.
tations of the sample are, both in terms 7. Kishor S, Johnson K. Domestic Violence in Nine
of its power to capture important asso- Developing Countries: A Comparative Study.
ciations and its generalizability. Calverton, MD: Macro International; 2004.
8. Ellsberg M, Peña R, Herrera A, Liljestrand J,
Winkvist A. Candies in hell: Women's experiences
of violence in Nicaragua. Social Science and
Medicine. 2000;51(11):1595-1610.
9. Ellsberg MC, Winkvist A, Peña R, Stenlund H.
Women's strategic responses to violence in
Nicaragua. Journal of Epidemiology and
Community Health. 2001;55(8):547-555.
10.Ellsberg M. Candies in Hell: Research and
Action on Domestic Violence in Nicaragua
[Doctoral Dissertation]. Umeå, Sweden: Umeå
University; 2000.
200 Researching Violence Against Women