اعداد د.علي الشعبي خبير الدراسات والبحوث في المجموعة
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الاتساق الداخلية
تستنتج الصدق عن طريق تصنيف أسئلة على هيئة استبانة بحيث تقيس نفس المفهوم . على
سبيل المثال ، يمكنك أن تكتب مجموعتين ، كل مجموعه مكونه من ثلاث أسئلة لكي تقيس
نفس المفهوم ( فلنقل مثلا مشاركة الطلبة في الفصل ) وبعد تجميع الإجابات ، قم بربط
اثنين من مجموعات الأسئلة لكي تقرر إذا ما كانت آلة المقياس تقيس المفهوم بمصداقية أم لا.
إن الفرق الحقيقي بين الصدق والثبات هو في الغالب مسألة تعريف. فالصدق يقدر مدى ترابط المقياس الذي تستخدمه ، أو ببساطة أكثر هو الدرجة التي
تقيس بها في كل مرة بنفس الطريقة المستخدمة تحت نفس الظروف مع نفس الأشخاص . الثبات
، من ناحية أخرى، تتضمن الدرجة التي تقيس عليها ما يفترض أن تفعله ، ببساطة أكثر ،
دقة قياسك . في اعتقادي أرى أن الثبات ( الدقة ) أكثر أهمية من الصدق لأنه اذا لم
تستطع أداة المقياس ان تقي
www.socialresearchmethods.net/tutorial/colosi/lcolosi2.htm
Reliability and Validity: What's the Difference?
Reliability
Definition: Reliability is the consistency of your measurement, or the
degree to which an instrument measures the same way each time it is used under
the same condition with the same subjects. In short, it is the repeatability of
your measurement. A measure is considered reliable if a person's score on the
same test given twice is similar. It is important to remember that reliability
is not measured, it is estimated.
There are two ways that reliability is usually estimated:
test/retest and internal consistency.
Test/Retest
Test/retest is the more conservative method to estimate reliability. Simply
put, the idea behind test/retest is that you should get the same score on test
1 as you do on test 2. The three main components to this method are as follows:
1.) implement your measurement instrument at two separate
times for each subject;
2). compute the correlation between the two separate measurements; and
3) assume there is no change in the underlying condition (or trait you are
trying to measure) between test 1 and test 2.
Internal Consistency
Internal consistency estimates reliability by grouping questions in a
questionnaire that measure the same concept. For example, you could write two
sets of three questions that measure the same concept (say class participation)
and after collecting the responses, run a correlation between those two groups
of three questions to determine if your instrument is reliably measuring that
concept.
One common way of computing correlation values among the questions
on your instruments is by using Cronbach's Alpha. In short, Cronbach's alpha
splits all the questions on your instrument every possible way and computes
correlation values for them all (we use a computer program for this part). In
the end, your computer output generates one number for Cronbach's alpha - and
just like a correlation coefficient, the closer it is to one, the higher the
reliability estimate of your instrument. Cronbach's alpha is a less
conservative estimate of reliability than test/retest.
The primary difference between test/retest and internal consistency
estimates of reliability is that test/retest involves two administrations of
the measurement instrument, whereas the internal consistency method involves
only one administration of that instrument.
Validity
Definition:Validity is the strength of our conclusions, inferences or
propositions. More formally, Cook and Campbell (1979) define it as the
"best available approximation to the truth or falsity of a given
inference, proposition or conclusion." In short, were we right? Let's look
at a simple example. Say we are studying the effect of strict attendance
policies on class participation. In our case, we saw that class participation
did increase after the policy was established. Each type of validity would
highlight a different aspect of the relationship between our treatment (strict
attendance policy) and our observed outcome (increased class participation).
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