A value of NULL indicates the value is unknown. A value of NULL is different
from an empty or zero value. No two null values are equal. Comparisons between
two null values, or between a NULL and any other value, return unknown because
the value of each NULL is unknown.
Null values usually indicate data that is unknown, not applicable, or to be
added at a later time. For example, a customer's middle initial may not be
known at the time the customer places an order.
Now I got more confused, since it seems some conflicting answers above :-)
Foxman, I guess your point is that data in my example is a classification
problem, since a ball is either from A or B; OK, let's forget about that one
.
In practice, we often have a typical null hypothesis; for example, to test a
treatment is effective, the null is not effective; to test if a person is
ill based on a blood test, the null is that the person is healthy, etc.
My question is: From a pure statistical theory po... 阅读全帖
现在在做单位的minitab training.这个null hypothesis 弄不明白。
null hypothesis 是说sample mean = popullation mean.
The p-value is the probability of obtaining at least as extreme results
given that the null hypothesis is true whereas the significance or alpha (α
) level is the probability of rejecting the null hypothesis given that it is
true.
问题: obtain at least as extreme results 是什么意思？
In any experiment or observation that involves drawing a sample from a
population, there is always the possibility that an ob... 阅读全帖
What's the difference?
I know that Null is Oracle means no set value and it's not comparable.
Null==Null in Oracle doesn't make sense.
While how different is it compared to Null in C?
In oracle, say select max,
col1 col2 col3
--------------------
a b 1
a b 2
c d 4
c d
-------------------
group by col1, col2, i want something like a/b will return 2, c/d return
null, since c/d has a null value.
"select col1, col2, max(col3) from table group by col1, col2" doesn't work.
Is ia any way to make the max function return null if there is a null value
in the column?
Thanks!
A'= the transpose of A?
the null spaces should be the same?
Ax=0 => A'Ax=0 => Null(A)\subset Null (A'A)
A'Ay=0 => y'A'Ay=0, i.e, (Ay, Ay)=0 => Ay=0
=> Null(A'A)\subset Null(A).
Type I / II errors depend on what is the null hypothesis... Let me use an
example. Suppose two manufacturers A and B produce similar ping-pong balls.
On average, balls by A have a higher average weight than by B. Now we have
100 balls, and we want test if these balls are from A or B (the truth is one
of them, say).
In this case, we can test Null Hypothesis the balls produced by A; or a Null
Hypothesis is Ball produced by B.
My question is: it seems we can flip a Null Hypothesis depending on what... 阅读全帖
should define a deault not null value, example:
create table #tmp (aaa int not null primary key)
insert into #tmp values(1)
insert into #tmp values(2)
alter table #tmp
add bbb int default 0 not null
the statement
"alter table #tmp
add ccc int not null"
will fail
下面这个例子里面 makenull函数传入一个ArrayList的instance, 在函数里把它
设为null, 但是函数返回后, main函数里的这个instance并不为null.
这是为什么阿?
public class test {
public static void makenull(java.util.ArrayList s){
s = null;
}
public static void main(String[] args) {
java.util.ArrayList s = new java.util.ArrayList();
test.makenull(s);
// after test.makenull function, s is NOT null
}
}
My suggestion is not to do this, but I think you can use use
your own null:
vector null;
vector::iterator nullIt = null.begin();
since nullIt is pointing to this special vector, it can be used as your
special
value for null for other vectors.
I would use hash to keep my data and use list to keep iterators if I have to
. To me, hash memory layout is more stable (not if it grows).
Null matrix in MATLAB contains all zeros, but it's not the same with the
empty matrix[], e.g. a=[]. You can use null matrix in this way: 0A=A0=0, 0
is the null matrix and A is any matrix. You can say the null matrix has a
property of commutative just like the identity matrix of I.
So I wonder it is a bug of my SAS. It did count null value as well. I set
the value to '00'X. Maybe it is not a good way to assign null value?
Actually I run following code first
count(distinct case when A in ('01',,'02') then B else null end) as ...
However SAS took null as a variable instead value. So I used following code
instead
count(distinct case when A in ('01',,'02') then B else '00'x end)
The code works on oralce database but not on SAS. :(
I have seen a similar one in a textbook.
There are two identically appearing bowls of jelly beans. Bowl 1 contains 60
red and 40 black jelly beans, and bowl 2 contains 40 red and 60 black jelly
beans. Therefore, the proportion of red jelly beans, p, for the two bowls
are
Bowl 1: p = 0.6
Bowl 2: p = 0.4.
One of the bowls is sitting on the table, but you do not know which one it
is.
You suspect that it is bowl 2, but you are not sure.
To test your hypothesis that bowl 2 is on the table, you sample... 阅读全帖
This is part of my trigger code:
.....
select @a = FLD_A,
@b = FLD_B,
@c = FLD_C
from inserted
insert into TBL_A
values('A', @a+','+@b+','+@c)
.....
Now the problem is FLD_B may not be populated and thus has a
NULL value in the table. the '+' operator returns NULL if a
NULL string is part of the concatenation. The only way to
get around is to add
if ISNULL(@b)
@b=' '
before the insert.
Is there any better way of doing it? There will be dozens of
fields like FLD_B, and I real
well, maybe this is useful for dating service, bf/gf search engine. :P
select a.ID, a.FirstName, a.LastName, b.ID, b.FirstName, b.LastName
from Male a, Female b
where ( a.GirlFriendID IS NULL and a.IsGay = FALSE and
b.BoyFriendID IS NULL and b.IsLesbian = FALSE)
试过下面的，不行。第一个非Null的数据Ntile就是6，我一共有10000+数据
select *,
(CASE WHEN PB IS NOT NULL THEN NTILE(100) OVER(ORDER BY PB ASC) ELSE NULL
END) AS Rnk_PB6
into #temp6
from #temp5
Let me simplify it further, and put some numbers:
Average weight of balls from A is 1. From B is 1.1 Their STDs are 0.1
Now we got a ball. Its weight is 1.05. We try to test if this ball is brand
A or B.
It seems me that we can test either of the following:
1) Null Hypothesis: the ball is from A.
or 2) Null Hypothesis: the ball is from B.
My question is: from a statistical view, there is nothing which would
dictate 1) or 2) should be Null Hypothesis. Either would be fine and can be
chosen depend... 阅读全帖
