Washington State
Institute for
Public Policy
110 Fifth Avenue Southeast, Suite 214 • PO Box 40999 • Olympia, WA 98504-0999 • (360) 586-2677 • FAX (360) 586-2793 • www.wsipp.wa.gov

June 2006

SEX OFFENDER SENTENCING IN WASHINGTON STATE:
DOES THE PRISON TREATMENT PROGRAM REDUCE RECIDIVISM?
The 2004 Legislature directed the Washington
State Institute for Public Policy (Institute) to conduct
a comprehensive evaluation of the impact and
effectiveness of current sex offender sentencing
policies.1 Because this is an extensive topic, we
are publishing a series of reports.
The Washington State Department of Corrections
(DOC) has operated a prison-based Sex Offender
Treatment Program (SOTP) at the Twin Rivers
Corrections Center since 1988. The program has
undergone a series of changes since its inception.
Since 1996, the program has used a combination of
treatment techniques including group therapy,
psycho-educational classes, behavioral treatment,
and family involvement. The length of treatment
has decreased from two years in 1996 to
approximately one year currently. Since 2000, sex
offenders assessed as having a high likelihood to
reoffend, based on their criminal history, are
prioritized for program entry.2

SUMMARY
The Washington State Department of Corrections
(DOC) has operated a prison-based Sex Offender
Treatment Program (SOTP) at the Twin Rivers
Corrections Center since 1988. SOTP uses a
combination of treatment techniques including group
therapy, psycho-educational classes, behavioral
treatment, and family involvement.
The purpose of this study is to estimate whether
SOTP reduces recidivism by comparing the
recidivism rates of sex offenders willing but not
participating in SOTP with those who did participate.
The study sample consists of sex offenders released
between January 1996 and December 1999 after
serving at least one year in prison.
Two methods of analysis are employed: logistic
regression for the entire study sample and logistic
regression for a risk-factor matched sample. Both
methods find the following:

Offenders selected for the treatment program must
meet the following five requirements:

• No statistically significant differences are found
between the two study groups for felony and
non-sexual violent felony recidivism.

• Sex offense conviction
• Voluntary participation

• The SOTP group has a statistically significant
higher felony sex recidivism rate than the
comparison group. However, the difference in
the felony sex recidivism rates between the
groups is small—less than two percentage
points.

• Admission of guilt
• One year minimum remaining in prison
• Medium or lower custody classification
This report estimates whether SOTP reduces
recidivism by comparing the recidivism rates of
sex offenders who were willing but did not
participate in SOTP with those who did
participate in the program.
A previous Institute report determined there are
significant differences between sex offenders who
participate in the SOTP and sex offenders not willing

This study finds that SOTP does not reduce the
recidivism rates of participants.

to participate.3 Because of these differences, the
comparison group for this study includes only those
sex offenders who indicated they were willing to
participate in the program.

3
1

ESHB 2400, Chapter 176, Laws of 2004.
2
The SOTP uses three risk for sexual reoffense
assessments: MnSOST-R, RRASOR, and Static 99.

R. Barnoski (2006). Sex Offender Sentencing In
Washington State: Who Participates in the Prison
Treatment Program? Olympia: Washington State Institute
for Public Policy, Document No. 06-06-1204.

Study Groups. SOTP staff indicated that the
program changed significantly in 1996, and any
evaluation should include only sex offenders who
have participated since that year. In addition,
measuring sex offender recidivism rates requires a
five-year follow-up period for reoffending and an
additional one-year period for the adjudication of
offenses.4
Thus, this outcome study examines sex offenders
willing to participate in the STOP who were
released between January 1996 and December
1999 after serving at least one year in prison. This
group meets the required six-year period to
adequately measure recidivism.5
Exhibit 1 displays the number of sex offenders in
the two study groups and their characteristics:
those participating in SOTP and those willing but
not participating. The SOTP group has 655 sex
offenders; 983 are in the comparison group.
The analyses reveal the following differences
between the study groups:
• The SOTP group includes slightly more
repeat sex offenders.
• SOTP participants have a higher percentage
of sex offenders with a prior conviction for a
child sex offense.
• SOTP participants spent a slightly longer
time in prison.
Based on these differences, one might expect that
the SOTP participants would be at a higher risk for
committing another felony than the comparison
group.

4

R. Barnoski (2005). Sex Offender Sentencing in Washington
State: Measuring Recidivism. Olympia: Washington State
Institute for Public Policy, Document No. 05-08-1202.
5
A 1994 study by the Institute examined sex offenders
who completed the program in 1993 and compared them
to a similar group who were released during the same time
period who did not participate in the program. The followup period was three years. The study examined rearrest
rates and found that program participants had slightly
lower rates for sex crimes (11 percent compared to 12
percent), violent crimes (1 percent compared to 3 percent),
and non-violent crimes (5 percent compared to 6 percent).
None of the differences were statistically significant,
meaning they could have occurred by chance. See L.
Song and R. Lieb (1994). Preliminary Recidivism Rates:
The Twin Rivers Sex Offender Treatment Program
(Revised). Olympia: Washington State Institute for Public
Policy, Document No. 04-06-1102.

However, when actuarial risk scores are applied
for these two groups, a different picture emerges.6
The risk scores are calculated using an actuarially
based static risk assessment tool being developed
by the Institute for DOC.7 The SOTP group
exhibits a slightly lower, yet statistically significant,
risk for reoffending.
Exhibit 1

Characteristics of Sex Offenders in Study Groups
Released From Prison Between 1996 and 1999
Sex Offender
Characteristic
Number of Offenders
Average Felony Risk
Score
Average Violent
Felony Risk Score
Percentage With Two
or More Felony Sex
Sentences
Percentage With
Prior Child Sex
Conviction
Average Years in
Prison
Average Age at
Release
Race/Ethnicity:
European-American
African-American
Native-American
Asian-American
Hispanic Origin

Study Groups
SOTP
Comparison
655
983

Difference
n/a

43.5

44.8

-1.3*

25.6

26.2

-0.6*

14.2%

12.9%

1.3%*

63.8%

50.3%

13.5%*

4.3

3.9

0.4*

38.6

39.3

-0.7ns

89.2%
7.9%
2.1%
0.8%
5.0%

78.8%
13.8%
3.4%
3.1%
13.1%

10.4%*
-5.9%*
-1.3%ns
-2.3%*
-8.1%*

* Statistically significant at the .05 probability level
ns
Not a statistically significant difference
n/a: not applicable

Exhibit 2 displays the five-year recidivism data for
the study groups. Of the 655 offenders in the
SOTP group, 82 recidivated with a felony, 27 with a
violent felony other than sex, and 12 with a felony
sex offense. These figures represent recidivism
rates of 12.5, 4.1, and 1.8 percent respectively.
The recidivism rates of the SOTP group are within
three percentage points of the rates for those in the
comparison group.

6

The comparison group includes all incarcerated sex
offenders who indicated a willingness to participate but
did not (willing, applied, declined, and rejected).
7
There is no static risk score for felony sexual
reoffending because criminal history alone does not
adequately predict sexual reoffending. The Institute’s
criminal history database is used to calculate these
scores.

Exhibit 2

Comparison of Study Groups’
Actual Five-Year Recidivism
Study Groups
SOTP

Comparison Difference

Number of Offenders
655
983
Number Recidivating Within Five Years
Felony
82
151
Violent Felony (Not Sex)
27
50
Felony Sex
12
6
Percentage Recidivating Within Five Years
Felony
12.5%
15.4%
Violent Felony (Not Sex)
4.1%
5.1%
Felony Sex
1.8%
0.6%

1,638
n/a
n/a
n/a
-2.8%
-1.0%
+1.2%

n/a: not applicable

Adjusted Recidivism Rates by Key Factors
While it is straightforward to compute the
recidivism rates of SOTP participants, the difficult
task is estimating what the recidivism rates would
have been if, keeping everything else the same,
these sex offenders had not participated in the
program. The ideal method is to randomly assign
a group of sex offenders to either SOTP or a notreatment comparison group. Under this optimal
research design, one can be quite certain that any
observed differences in recidivism rates between
the treatment and comparison groups is due solely
to the effect of the treatment.
However, a random assignment design cannot be
used since the task is to retrospectively evaluate
SOTP. Therefore, two alternative approaches are
used to evaluate the impact of SOTP on recidivism:

Method 1: Standard Logistic Regression. This
approach uses logistic regression to estimate
whether SOTP participation affects recidivism by
statistically controlling for systematic differences in
offender characteristics between the two study
groups. These characteristics include age,
gender, ethnicity, prior criminal convictions, and
the defendant’s current charges.
Method 2: Risk-Factor Matching. In a second
approach, we create a comparison group of sex
offenders who did not participate in SOTP but
have characteristics matched to the participants.
A comparison group is chosen by finding
individual sex offenders with risk factors that
match specific SOTP participant risk factors. The
result is a one-to-one match between a SOTP
participant and a non-participant where both
offenders have the same risk factors.
Exhibit 3 presents the results of Method 1, the
standard logistic regression analyses. The exhibit
displays adjusted recidivism rates. These
represent the recidivism rates assuming all the
offenders have the same risk factors. The
statistical adjustments reduce the differences in
recidivism rates between the two groups.
There are no statistically significant differences
between the two study groups for felony and nonsexual violent felony recidivism. The SOTP group
has a higher felony sex recidivism rate that is
statistically significant, although the difference is
only 0.8 percentage points.
Exhibit 3

• Standard logistic regression, and
• Risk-factor matching in combination with
logistic regression.
The SOTP evaluation is a challenge because of
the self-selection process for participation in this
program. Factors we cannot measure in this study
may influence an offender’s decision to participate
in the SOTP. As a result, SOTP participants may
have recidivism rates different than the comparison
group not necessarily due to the effects of the
treatment, but due to the factors that resulted in the
offender deciding to enter the program.

Standard Logistic Regression:
Adjusted Five-Year Recidivism Rates
Type of Recidivism

SOTP

Comparison

Difference

Number of Offenders

655

983

1,638

Felony

9.2%

10.4%

-1.2% ns

Violent Felony (Not Sex)

3.7%

3.8%

-0.1% ns

Felony Sex

1.3%

0.5%

+0.8%*

* Statistically significant at the .05 probability level
ns
Not a statistically significant difference

Exhibit 4 displays the actual five-year recidivism
data for the two risk-factor matched groups. Of the
655 SOTP participants, 432 sex offenders were
matched with those who indicated a willingness to
participate but did not (66 percent). The matching
by risk factors means there are no differences
between the groups on these variables.

felony sex recidivism rate.8 As a result, felony sex
offenses are included in the violent felony rate.

Of the 432 offenders in the SOTP group, 46
recidivated with a felony, 17 with a violent felony
other than sex, and 8 with a felony sex offense.
These numbers result in recidivism rates of 10.6,
3.9 and 1.9 percent respectively. The +1.6
percent difference in felony sex rates between the
SOTP and comparison groups is statistically
significant.

Logistic Regression Results
Adjusted Five-Year Recidivism Rates

There are no statistically significant differences
between the two study groups for felony and violent
felony recidivism.
Exhibit 5

Type of
Recidivism

SOTP

Comparison

Difference

Felony

6.2%

6.7%

-0.4%ns

Violent Felony

4.4%

2.6%

+1.8%ns

Felony Sex

n/a

n/a

n/a

ns

Not a statistically significant difference
n/a: A recidivism rate cannot be estimated because only one
sex offender in the comparison group recidivated with a
felony sex offense.

Exhibit 4

Risk-Factor Matched Sample
Actual Five-Year Recidivism
Study Groups
SOTP
Number of Offenders

432

Comparison Difference
432

Number Recidivating Within Five Years
Felony

46

49

-3

Violent Felony (Not Sex)

17

14

+3

Felony Sex

8

1

+7

Percentage Recidivating Within Five Years
10.6%

11.3%

-0.7%ns

Violent Felony (Not Sex)

3.9%

3.2%

+0.7%ns

Felony Sex

1.9%

0.2%

+1.6%*

Felony

Findings
The purpose of this study is to estimate whether
SOTP reduces recidivism by comparing the
recidivism rates of sex offenders willing but not
participating in SOTP with those who participated.
Two methods of analysis are employed: logistic
regression for the entire study sample and logistic
regression for a risk-factor matched sample. Both
methods find the following:
• There are no statistically significant
differences between the two study groups
for felony and non-sexual violent felony
recidivism.

* Statistically significant at the .05 probability level
ns
Not a statistically significant difference

• The SOTP group has a higher felony sex
recidivism rate than the comparison group
that is statistically significant. However the
difference in the felony sex recidivism rates
between the groups is small—less than two
percentage points.

Exhibit 5 displays the adjusted five-year recidivism
rates for the risk-factor matched cases using logistic
regression as in the standard regression method.
Because only one sex offender in the comparison
group recidivated with a felony sex offense, it is not
possible to calculate an adjusted
8

Five comparison group sex offenders who recidivated
with a felony sex offense were not matched to the SOTP
group.

For further information, contact Robert Barnoski at
(360) 586-2744 or barney@wsipp.wa.gov

Document No. 06-06-1205

Washington State
Institute for
Public Policy
The Washington State Legislature created the Washington State Institute for Public Policy in 1983. A Board of Directors—representing the legislature,
the governor, and public universities—governs the Institute and guides the development of all activities. The Institute’s mission is to carry out practical
research, at legislative direction, on issues of importance to Washington State.