 This is also very important to understand the key concept which are used in test and measurement. So, jyapta kame un terms ki complete understanding nahi hogi, it would be very difficult for us to comprehend those technical concepts. Many important technical concepts necessarily came into plan when attempting to understand the proper use of psychological tests. Like agar ek hum assessment karein, test karein and we are not aware of the time that either it is valid and reliable and what does reliability and validity means, to us test ko hum usko use bhi nahi kar sakte, because we have to claim certain things before using a psychological test. This includes the concept of reliability, validity, sources of error and standard errors of measurement. So, we have to see ke kya humara test to hai wo in sabteero se huzar kar ek usable instrument ban chuk hai ya nahi. So, talking about the reliability as a first concept. Reliability refers to the property of repeatable results. Ab reliability kya hai ke jab hum ek test to hai usko ek population pe mayar karein hai, to kya across the time span is going to mayar the same thing in an accurate manner or not. Tests of relatively stable phenomena, for example, should have high test detest reliability. Agar wo trade jisko hum mayar karein hai, wo ek assi state hai, jo ke ek permanent ek position rakhti hai ki individual mein, tisne hum intelligence ki baat karte, to intelligence to hai, it is relatively a stable mayar, to usko hum jitni bhi dafa jo bhi test apply karein ge, do intelligence ka score more or less same hi aega. Agar ko individual jo hai, wo intellectually jo hai, wo low hai aur uska jo IQ level hai, wo below 70 hai, wo ap uski po jo bhi intelligence ke test apply karein ge, wo reliable hongi aur wo sub test usko ek below the average intelligent person hi reflect karein. So this is the case ki jaha par hum liability ki baat karein ki reliability is something disko hum repeat bhi karte hai, to it gives you the same results. For example, if a test is designed to mayar a trade such as introversion, then each time the test is administered to the subject, the result would be approximately same. Agar hum koi introspective behavior dekh re hai, hum apna within stealth dekh re hai, ki ek individual jo hai, uske andar kitani zyada wo honesty, to wo honesty jo hai, wo uske introspective observation, uske behaviori assessment mein poha zyada grossly change nahi hogi, wo virtue uske andar jitna hai, wo usko jitni bha bhi aap test karein ge, wo apko utna hi valid aur utna hi reliable aapko nazar aega. Then comes to the reliability, validity. Validity is the extent to which the test mayar, what it claims to be mayar. I mean, whatever we are claiming that this test is going to mayar a certain phenomena that is exactly mayaring the same one, it is vital for a test to be the valid in order for the result to accurately applied and interpret. Agar hum kite hai, ke haan ek individual jo hai uske andar shame and guilt ka component mayar karein. Lekin wo test actually shame and guilt mayar nahi kaha, wo basically shayad anxiety aur depression ka scale hai. So, it means that this is not a reliable and valid mayar, exclusively it is not a valid mayar to mayar the certain properties, so it should mayar what it claims to mayar. For example, a test might design to mayar a stable personality trait, but instead mayar transitory emotions generated by situational or environmental conditions. We claim to yeh karein ki yeh jo test hai, yeh let's say ke ek permanent jo hai, wo aap ki state ko mayar karein. Let's say, hum kitein ki yeh aap ke mood ko mayar karein. Lekin wo jo test hai uske items hai, wo aap ke temporary mood ko dekhne hain ke, is waak iss situation ke andar aap ko kesa lag raha hai. So, may be wo jo aap ke emotions hai, wo jo aapka mood hai, wo true or valid picture nahi hai, aapki permanent state of mood ki. So, to become a test as a valid mayar, it should mayar what it claims to mayar. Hum usko thoda sa twist nahi karsate uske instructions ko hum manipulate nahi karsate uske construct ko hum derail nahi karsate. A valid test ensure that results are the accurate reflection of their dimension undergoing assessment. Like for instance, agar humana scale ye mayar kare hain ki wo ek specific trait aggression ko mayar kare hain, to it should only mayar aggression yet not abuse.